Johnny Nelson has warned Kell Brook he could get "pick-pocketed" by Kevin Bizier if he is not fully focused on their world title fight.
The IBF welterweight champion makes his mandatory defence against the Canadian in Sheffield on March 26, live on Sky Sports 1 HD.
Brook (35-0-KO24) missed out on a Wembley battle with Amir Khan and earlier this week took another swipe at his bitter British rival.
Nelson was with Brook earlier this week and made it clear that the 'Special One' needs to shake off the Khan disappointment, put on a show against Bizier and retain his title.
"To me he [Brook] is top of the welterweight tree," Nelson told Sky Sports News HQ.
"But the problem is now Amir Khan has stolen the headlines and Kell thought that was his summer showdown - against Khan - but now he's just got to take something that's not floating his boat.
"He's got to lift himself and think to himself: 'I know this guy has trained very hard for me. I can't get complacent and take it easy'.
"If he does, Bizier is a busy fighter that could pick-pocket him. My worry is Kell needs that challenge to keep him busy, keeps him frightened and keeps him in the gym.
"If he thinks it's a push-over, he starts to cut corners, take it easy and drop his guard a bit."
Bizier (25-2-KO17) has worked his way into the mandatory position but Brook is a huge favourite to make it 36 wins out of 36 in front of his local support at the Sheffield Arena.
The champion has already promised to put on a show and make the rest of the welterweight division take note, with WBA champion Keith Thurman rescheduling his defence against Shawn Porter to June 26 after being injured in a car accident.
Brook beat Porter in California to become the world champion in August 2014 and has made two one-sided defences since.
His first came against Jo Jo Dan and although the Romanian has beaten Bizier twice in the last two years - his only defeats as a professional - Nelson is hoping the results will not influence Brook's preparations.
"Kell will be thinking Jo-Jo Dan beat him twice, so he might be thinking this is really easy, but it's not as easy as it looks," he added.
"Kell has to keep on top of him, close him down, get rid of him and make a statement. Not just win."