Paul Scholes is set to be ruled out for United until into the New Year, reports the Manchester Evening News.
Sir Alex Ferguson reported that Scholes had felt a 'click' in his knee during the session in Dynamo's Olympic Stadium.
United couldn't fly their midfielder home early but after returning from the Ukraine in the early hours of Wednesday morning, he was sent for a scan.
Reds boss Fergie said the results didn't show clearly the problem but Scholes had a further appointment to try to ascertain the damage.
But there are serious fears that Scholes will be told he will be out for three months, leaving him looking at a return to fitness in February.