Radamel Falcao is not the player he was before his serious knee injury, says former Chelsea striker Hernan Crespo.
Falcao is on loan at the Blues from Monaco after a poor spell at Manchester United following his recovery.
"There is a big difference between Falcao before and after his injury," Crespo told BBC World Football.
"Before the injury, he was the best of the 'human players' - all players apart from Lionel Messi and Cristiano Ronaldo, who are on another level."
Falcao was one of the highest-rated strikers in the world, scoring 62 goals in 72 games for FC Porto and 68 in 86 for Atletico Madrid before his £50m move to Monaco in 2013.
But he suffered anterior cruciate ligament damage halfway through that season and ended up missing the World Cup.
He spent last season at Old Trafford, but only scored four goals in 29 appearances and United boss Louis van Gaal opted not to sign him.
Crespo said the key for Falcao, who has joined up with his new team-mates in Montreal, was persuading Blues boss Jose Mourinho to play him.
"After such an injury, you need time and it's not easy to recover," added Crespo, now manager of Serie B side Modena.
"It won't be easy for him. Jose doesn't give presents away.
"All Falcao has to do is to believe in himself and be the great player he is. That will be enough to convince Mourinho to give him a chance.
"He was probably not the kind of player Van Gaal was looking for," added the 40-year-old Crespo.
"You have to take your chances if you want to play regularly. We will see what happens at Chelsea."
You can hear more from Crespo by listening to this week's episode of World Football on the BBC World Service.