The Colombian striker arrived in a deadline day £6m loan deal over the summer, with the Red Devils having put him through a strict medical.
Falcao had missed most of 2014, including to the World Cup, after injuring his knee while playing for Monaco, which is understood to have put other parties off signing him.
After an initial bright start at Old Trafford amid speculation that his ACL problem had cleared up, the South American has been sidelined in recent weeks.
It has now emerged that the attacker has ice strapped to the troublesome joint after games and training sessions, which has raised concerns among his team-mates.
And now members of the United hierarchy are also worried, with a permanent deal, which clauses are in place for, set to cost a total of £52m, plus his £280,000-per-week salary.
At 28 there are also worries that the prolific marksman will be unable to ever fully recover from the issues he currently has with his knee.