Everton manager David Moyes fears fatigue could damage his side's chances of qualifying for next season's Champions League.
They have not been out of the top six since September but have won only two of their last seven Premier League games, and Moyes has said he believes the demands of a tough domestic season are starting to take their toll on his relatively small squad.
He told the Liverpool Daily Post: "You could see tiredness in people like Leon Osman, Steven Pienaar, who we rely on heavily. We're asking people like Leighton Baines to put in a big shift, to do it for us week in, week out.
"I've always said maybe it's because the numbers that we've got only allow us to have maybe a good half season, not a full one, because we're asking a small number of players to do that for a full season.
"I hope I'm wrong, because we tend to finish the second half of seasons very strongly. We've had a really good opening half of the season, and we've got to hope we can do it again."
Everton's cause at Old Trafford was not helped by the loss of central defender Sylvain Distin, who pulled out of the game because of illness just 35 minutes before the 4pm kick-off.
"I didn't know until about 3.25," Moyes said. "It was maybe five or ten minutes before they were due to go out for the warm-up.
"He'd felt sick during the night but really thought he was going to be all right, but probably felt unwell when he started to get ready."