The Red Devils got the better of their local rivals thanks to an impressive opening 30 minutes, much to the former midfielder's surprise
Roy Keane was pleasantly surprised by Manchester United's "outstanding" display in Saturday's 2-1 win at Manchester City.
The Red Devils took a two-goal lead within the first 30 minutes against the reigning Premier League champions through Marcus Rashford's penalty and a goal from Anthony Martial.
Nicolas Otamendi pulled one back for City with five minutes left, but United held on for a second straight league win, having beaten Tottenham 2-1 in midweek.