Ferguson was forced to play his fringe players as injuries bit in November and December and the side went 10 games without defeat, including victories over Liverpool and Arsenal.
But the return of David Beckham, Roy Keane, Rio Ferdinand and Laurent Blanc has seen United lose their last two games to Blackburn Rovers and Middlesbrough.
"I have only 12 players to pick from, then when all the players come back, we start losing," Ferguson told reporters on Friday. "Whether it is coincidence or not - it is strange. It is frustrating to lose these games, but it was two difficult away games.
"A lot of teams' home form has been very good this season. We have been caught on counter-attacks twice. We were in complete control of both matches. When you are in control of games, you have to make sure you have the patience to play."
United have a chance to return to winning ways on Saturday when ex-Old Trafford captain Steve Bruce brings his Birmingham side to his former employers.
"Steve Bruce has done very well," Ferguson said. "There is a lot of enthusiasm about him. He was the same as a player. He was bubbly and I'm sure his team is reflecting that. Birmingham are certainly determined.
"They have a lot of energy and have a great chance of staying in the premier league because they are consistently getting decent results.
"Any team who comes out of the first division always finds it difficult. There have been one or two exceptions like Ipswich, but even they went down the following season.
"It is important to get off to a good start which Birmingham did. Hopefully, they gather experience as they go along. If you have a bad start, you don't have the experience to recover.
Ferguson may rest Keane to allow him time to catch his breath after playing in two games since returning from a hip operation.
Defender Mikael Silvestre is also struggling and the United manager may consider changes to freshened up his side.
"We have to win the next two games," Ferguson said. "Home form is going to be important. I look at the side in terms of freshness. See if we can bring in one or two to make it fresher. We will have to wait and see how Roy Keane is after his exertions yesterday (Thursday).
"It was his first full game at that level. He did remarkably well but we will have to see how he has come through that."