The Belgian winger has made just six substitute appearances this season, playing only 122 minutes of Premier League football.
"That Adnan had to prove himself for Louis van Gaal in that match is nonsense," he is quoted as saying by the Daily Star.
"After an elbow in training Adnan's mouth was ripped open and he was under antibiotics and really should not have played.
"But after 20 minutes Shinji Kagawa is injured and Adnan suddenly needs to play. Then it makes sense that you do not play your best game ever."
Januzaj scored four goals in two appearances for United's Under-21 side last month.