Kevin Sinfield called Rob Burrow "the toughest and bravest man I have ever met" as he paid tribute to his friend and former Leeds Rhinos team-mate after his death at the age of 41.
Leeds announced on Sunday evening that their former scrum-half had passed away after a four-and-a-half-year battle with motor neurone disease.
Sinfield and Burrow have raised more than £15m for MND charities since Burrow's diagnosis in 2019, two years after he retired from playing.
- Rugby league legend Rob Burrow dies at age of 41
- Rob Burrow obituary: Rugby league icon, MND campaigner
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The Rugby Football League (RFL) has said there will be a one-minute silence at the Challenge Cup final at Wembley Stadium on Saturday, plus applause on the seventh minute in both the men's and women's finals.
Burrow famously wore the No 7 jersey for Leeds.
Sinfield wrote on social media: "Today was the day that I hoped would never come.
"The world has lost a great man and a wonderful friend to so, so many.
"You fought so bravely until the end and became a beacon of hope and inspiration, not only for the MND community but for all those who saw and heard your story.
"My love and thoughts go to your beautiful family, Lindsey, Macy, Maya and Jackson, to your lovely parents Geoff and Irene, sisters Joanne and Claire and your wider family and friends.
"You will continue to inspire me every single day. I have lost a dear friend and I will never forget the special times we shared both on and off the pitch.
"I would always say that you were pound for pound the toughest player I ever played alongside, however since your diagnosis you were the toughest and bravest man I have ever met.
"The last four and a half years you showed the world what living and loving looked like and this was always done with the biggest smile on your face.
"I will miss you my little mate."
In December last year Sinfield ran seven ultra-marathons in as many days across seven different cities, crossing the finishing line at The Mall in London after 27-mile routes in Leeds, Cardiff, Birmingham, Edinburgh, Dublin and Brighton.
'Love and admiration I have for Rob couldn't be any higher'
Speaking on Monday as building on the Rob Burrow MND treatment centre in Leeds began, Sinfield said: "So brave. So tough. I wish he could have seen the outpouring of love. I think we all lose special people but it is very, very rare that you lose someone who is so special to so many people.
"You can see it is pretty raw still. He leaves a massive hole and there will be a lot of people who are heartbroken. A lot of people from the MND community will be heartbroken. But he cared about everybody and what is really, really important is that Rob Burrow continues to live forever.
"Our team, the MND community, and everyone who has supported us previously will make sure that Rob's name is at the forefront of everything that we do going forward.
"To see what he has done in the last four and a half years. I was amazed at some of the things he did on the rugby field but to see some of the things he has done in the last four and a half years has been unbelievable.
"The respect and love and admiration I have got for him couldn't be any higher. The inspiration he has given everyone across the UK from this story, it will live in the memories forever.
"To see some of the fruits of Rob's work - he was so proud of this. I've heard they are going to try and build it in record time now, because that's what Rob did. He broke records.
"He wanted people to have a better outlook on life, he wanted people to have hope, he wanted find a cure for this disease. Building this centre and the work he has been able to do will ensure those that come after him will have a far better journey.
"If you don't remember Rob Burrow as a rugby player, go and take a look. You would be amazed at the guy. He was superhuman, superman in a game of big strong athletes.
"I am sure there will be a big photo of Rob in this building and I'm sure there will be kids that walk through with parents, grandparents and relatives and ask, 'who's that?'
"I'm very, very sure the answer we will get is, 'he was the legend behind this building'. That is some legacy for him to leave. He was a special guy.
"He will be looking down on us now and be so proud of his family for being here. He will be smiling like he always did."
Wife of Rob Burrow: He was loving, kind and caring
The wife of Rob Burrow has described him as "loving, kind and caring" and vowed to keep his legacy alive, one day after his death was announced.
In a moving tribute, Lindsey Burrow said she and the family were all "deeply saddened by the loss".
In a statement released through the Rhinos, his wife said: "I was incredibly proud and fortunate to call Rob my husband.
"I am unbelievably proud of the campaigning he's done to raise awareness and the millions of pounds that have been raised in his name for MND charities.
"I would like to thank the Rugby League community and everyone for their outpouring of love and support since Rob's diagnosis. I truly appreciate every message of support, and fundraising that has been done.
"My priority is to make Rob proud, and to bring our three children up as Rob would want and ensure their happiness and well-being.
"We will continue to keep Rob's legacy alive. We will continue to 'bang the drum' and do our best to try and help others.
"Although we knew this day would arrive, I am somehow still feeling at a loss for words that our loving, kind and caring husband and father has departed.
"However, we take comfort from how much people's love and continued support meant to Rob through his most vulnerable times.
"He was simply the best."
Peacock on Burrow: A peerless giant in terms of character
Jamie Peacock, another of Burrow's former Leeds team-mates, wrote on Instagram: "We all knew this day was coming, it was always going to be too soon and none of us wanted to think about it.
"Rob in a sport where courage, bravery and grit are characteristic of every player at the absolute top, you were the most courageous, the bravest and the grittiest of them all.
"And a sense of humour to match, I'll always remember you holding court in a dressing room having men twice as big cower because of your take no prisoners sense of wit.
"The world outside of Rugby League got to witness first-hand your courage, bravery, grit and sense of humour in the most difficult of situations in your inspirational battle with this brutal disease.
"Becoming an inspiration for 1000s and a North Star for the MND community, providing hope where before there was none. Never once complaining about your condition and situation.
"Small in physical stature but a peerless giant in terms of character, heart, spirit, resilience, courage, determination, humour and bravery.
"An inspiration to us all. RIP mate."
Jones-Buchanan: Rob was a superhero that transcended rugby league
Sky Sports Rugby League pundit and Burrow's former team-mate Jamie Jones-Buchanan said: "It is an incredibly difficult time at the minute. The love, the sentiment, that has been pouring in is a reflection of Rob's life and the legacy he has left behind.
"He has not just transcended rugby league but the sport, the nation and he gave out a message.
"I think for me, it is the word courage that stands out. Rob's courage to play the game with his size and his stature, something that people at the start thought was a weakness and he showed it was a strength.
"He was world class and he was a super hero, he inspired so many young people to play the game and had some incredible moments. Then that courage shone even brighter post-career.
"That courage will continue to be the centre of what blue and amber jerseys and Leeds Rhinos, sport, the nation means, shining the light on MND and the people in the community that need it most.
"Rob was a character right at the centre of the changing rooms, the camaraderie, the friendship, the cheeky character that he was. He led in the changing rooms, he led out on the field, and he is a superhero.
"Rob lived one, two, three lives fulfilled and we have to keep the mantle going now."
'Burrow taught us we must dare to dream'
Figures in rugby league and across the wider sporting world, as well as the Prince of Wales, were among those to pay tribute to Burrow, who played almost 500 games for Leeds during a 17-year career.