Will Greenwood has admitted the loss Billy Vunipola to injury is a huge blow to the British and Irish Lions' hopes in New Zealand.
The Saracens and England No 8 has been managing the injury but now requires further medical treatment following a brutal Aviva Premiership semi-final against Exeter Chiefs on Saturday.
The younger of the two Vunipola brothers has proved a vital asset to both club and country this year, and missed four months of the season with a knee injury before returning to help England retain their Six Nations crown.
He was man of the match in Saracens' Champions Cup final win over Clermont on May 13, and was a stand-out performer in Sarries' semi-final defeat at Sandy Park leading to speculation he would start the first Test against New Zealand on June 24, live on Sky Sports.
"His ball-playing ability is second to none in that back row," Greenwood told Sky Sports News. "You kick long to him, he comes back and he either charges through five or six people or, as he did in the semi-final of the European cup, he catches and fizzes 20-yard passes off his left or his right.
"Brilliant at the base and immensely fit considering looking at the size of the man and thinking surely he can't get around.
"Even when he was exhausted, and now we are finding out playing with one arm against Exeter, he was potentially the man of the match in that game and he could have been man of the match in potentially every game he has played in the past two years.
"It is a major blow personally for him as everyone wants to wear that red jersey, but he feels that he can't do his best carrying that knock."
Wasps' James Haskell has been called up as Vunipola's replacement and will join up with the squad after next week's Aviva Premiership final.
Haskell's inclusion adds to a host of back-row options available to the Lions, including Welsh duo Taulupe Faletau and Ross Moriarty and Ireland No 8 CJ Stander, but despite the calibre of talent at Warren Gatland's disposal, Greenwood believes the loss of Vunipola could have a large impact on the tour.
"There are options," Greenwood added. "You have to get over the initial disappointment of losing someone like him and then begin to understand this is a Lions tour, it is much more about the collective than the individual."
He added: "You start to remove the best of the best and you just start to impact your ability to compete at the highest level against the best team in the world."