The Daily Mail says Falcao's long-term fitness has been plunged into further doubt after it emerged that the United striker has to pack his troublesome left knee in ice after every game.
United insisted on a rigorous medical before signing Falcao from Monaco in a £6million loan on deadline day due to the cruciate ligament injury he suffered in a French Cup tie in January that ruled him out of last summer's World Cup.
The deal could rise to £52m if United decide to sign the Colombia striker permanently next year, but there are growing concerns at Old Trafford about committing to such a huge deal for a player who earns £280,000-a-week and has yet to prove his fitness.
Falcao's new team-mates are understood to be surprised that he has to wrap his knee in an ice-pack after matches and training sessions to prevent any swelling, and questions have also been asked about his ability to return to peak condition after such a serious injury at the age of 28.
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