Mourinho has opted for both Nemanja Matic and Marouane Fellaini ahead of the pair but he said Carrick and Herrera would get their chance.
"For Carrick and for Ander, it's just patience because their time will arrive," Mourinho told the club's website (www.manutd.com).
"They are going to be important players for us, for sure. That's the team, that's the squad. We cannot play with (only) 11 guys," the manager added.
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Matic, who signed from Mourinho's former club Chelsea in the close season, has made an immediate impact and was named man of the match in 4-0 wins over West Ham United and Everton while Fellaini has played well as a starter or off the bench.
Mourinho said his players must come to terms with squad rotation.
"Sergio Romero played the Europa League final, which was the game of the season for us, and he is not playing now. Herrera was player of the season and he is not starting matches. You know, that's life at big clubs," he said.
"We gave Michael Carrick a contract because he's an important player. He's just in a team where Matic and Fellaini are in the best form that I have ever seen them."
With Paul Pogba nursing a hamstring injury and Mourinho set to give Matic a rest, Carrick and Herrera could be given their chance in the League Cup game against Burton Albion later on Wednesday.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Peter Rutherford)
