Former Chelsea striker Hernan Crespo says striker Falcao is not the player he was before suffering anterior cruciate ligament damage at the start of 2014.
The Colombia international required surgery after suffering a very serious injury during Monaco's Coupe de France tie against Monts d'Or Azergues in January of last year.
He missed the World Cup Finals in Brazil but still managed to attract interest from Manchester United where he spent last season on loan.
He will spend next season on loan at Chelsea but Crespo says it will not be easy for the 29-year-old to rediscover the same form that made him one of Europe's best striker when at Atletico Madrid and Monaco.
"There is a big difference between Falcao before and after his injury," Crespo told the BBC.
"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.
"After such an injury, you need time and it's not easy to recover.
"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.
"You have to take your chances if you want to play regularly. We will see what happens at Chelsea."