The Red Devils twice surrendered the lead, with Ruben Amorim’s Milan side eventually completing a convincing comeback.
Carrick admitted the performance fell below expectations but remained optimistic ahead of United’s Premier League opener against Hull City.
“Today, we're disappointed with it. We didn't play well today. There are reasons for that and we need to improve,” he told the media.
“Some of the boys were playing their first minutes. Different reasons - but no excuses. We didn't play very well,' Carrick told reporters following the final whistle.
“Overall, we are ready for next week. A lot of the boys will be better off for the minutes. We've done a lot of work on the training pitch, the performances in the games have been pretty good. We're still in good spirits going into next week.”