"I trained alone (at Chelsea). I can feel it in some parts of the game," Matic said.
"My body is not ready yet and I have some difficulty, of course, but I try to hide that and to play simple. I will be better."
The 29-year-old's next game for United is a visit to Liverpool and says his side are only focused on that match and not the title race.
"We know what we have to do because we are Manchester United and we are focused just on the next game," Matic added.
"Always the next game is the most important, so game by game we have to collect points and next May we will see. It's a long way to go.
"As you know, we've won almost every game, we only drew at Stoke - in the Premier League it is very hard to do this.
"This can only be done by big teams, so we showed that we are a big team and we have to continue like this.
"We cannot be relaxed for the future because we have to win more, focus because the teams now know how we play.
"The teams know our formation, our players and, of course, in the future those will want to show they can play against Manchester United so we will be focused, we will be ready for that. It is going to be hard but that's why we are here."