The Red Devils were held to a 0-0 draw at Anfield this afternoon in a dull affair.
United had just one shot on target and failed to cause Liverpool many problems.
And Mourinho says his players, who returned from international duty this week, were fatigued against the Reds.
"The opposition was good today, but we played our game," Mourinho said.
"In the first half we had a couple of very good chances and good control of the game.
"In the second half Matic tired and I had no solutions on the bench.
"I tried with my substitutions to make us more dynamic in attack, but in reality we lost power and energy and control in midfield.
"We had one shot on goal - well for you and the experts it is easy to speak but it is more difficult from the sidelines.
"Liverpool's midfield was really strong. They are faster than us when the game breaks.
"I had no chance to bring the game in another direction so it is a positive point."
The point sends United to the top of the Premier League ahead of Manchester City.
However, Pep Guardiola’s men host Stoke this afternoon and can regain top spot.
