The Scottish manager claims that the England international is still not quite at peak condition despite playing a key role in Saturday's 4-3 victory over Reading
Despite putting in his one of most impressive display of the season so far against Reading, the Red Devils boss believes that the 27-year-old needs more games to reach his peak and plans to field him against Cluj on Wednesday.
"Wayne needs to play a lot of games to get his match fitness up. He's a strong-built lad and when he misses two games it takes him three games to come back," Sir Alex told reporters on Tuesday.
"It's the way he's always been. He'll play tomorrow night and hopefully that brings him on further. On Saturday, that was his best performance of the season so far."
The Scot challenged the former Everton player to improve his goalscoring record on Friday and Rooney responded with a key role in United's thrilling 4-3 victory against Reading, with two goals and an assist for Robin van Persie's winner.
With the Manchester derby to come on Sunday, Sir Alex has confirmed his intention to give chances to some fresh players in Wednesday's Champions League tie, while ruling Nemanja Vidic out of both that game and the Manchester City clash.