Diarra has had to wait for playing time this season with Xabi Alonso and Sami Khedira being Mourinho's preferred midfield pairing, but with the latter out injured, the France international took his chance to impress.
"Lass was the man of the match. He was incredible in every aspect," said Mourinho.
"We have several injured players and these two are now suspended. Khedira will return. This is something normal to deal with at the end of the year.
"People might think that we could drop points due to having so many players out, but we'll see. We'll put 11 players on the pitch and they'll have the support of the fans. We want to go into the Christmas holidays with a win.
"We must go into the two games we have left (Madrid also play Levante in the Copa del Rey) with all the strength that we have. We also hope that Kaka will return from January."