United meet Crystal Palace on Thursday evening in the Premier League, while Chelsea enjoyed a narrow 1-0 win over Norwich on Tuesday, giving them an extra 48 hours to prepare for Sunday's match at Wembley.
"There is a concern, obviously, that they [Chelsea] will have had 48 hours' more rest and recovery than us. It's not fair," the United manager said.
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"We spoke about a fair scheduling going into this restart and, of course, it isn't. But I have to think about Thursday, winning that one [against Palace], focusing on that one, and then pick up the pieces.
"It's not going to be a problem and a concern for us, but 24 hours, 48 hours is a big difference at this time [of the season] so we've not been handed four aces, to put it that way, for the last two weeks.
"But we're going to have to put on a poker face and play the cards well. The next two weeks will be hectic but we're very fit - our lads have not felt as fit as this for years."
Arsenal and Manchester City meet in the other FA Cup semi-final, which takes place on Saturday. Both teams played their most recent Premier League games on Wednesday.