We are set to lose Park Ji-sung for up to four weeks after he tweaked a hamstring in training before the Derby.
The 29-year-old South Korean has only just returned following his international involvement.
Park, who picked up his 100th cap against Japan, has since retired from international football to concentrate on his United career.
"We got a blow on Saturday morning with the news that Ji-sung Park did his hamstring on Friday afternoon in training," the boss explained to ManUtd.com. "It was very unfortunate because it was his last kick of the ball in training as well. We were really looking forward to having him back after being away for a month at the Asian Cup. He’ll be out for up to a month, which is a blow to us."
Defender Jonny Evans has had a scan on an ankle injury and could be out for a month also.
But on the other hand, Antonio Valencia is back training with the reserves. The winger faced ligament damage in an innocuous challenge during United's 0-0 Champions League draw with Rangers at Old Trafford in September.
"The good news is that Valencia started training with the Reserves on Saturday," added the boss. "So he’s on his way back and that will be a real boost to have him available for the run-in."