The former Arsenal defender feels the 19-year-old needs to improve on a key aspect of his game.
Chris Smalling's sending off before the break proved a heavy burden for Louis van Gaal's side, although they have received plenty of praise for their work-rate and determination after going down to 10 men.
United did not have enough about them to stop the Premier League champions, though, and Keown has singled out young defender McNair.
The ex-Arsenal man feels the Northern Irishman 'switched off' for Sergio Aguero's goal and says it is 'important' the youngster works on getting touch-tight to the man he is marking in the box.
"The young lad Patrick McNair switched off for the goal," Keown wrote in his weekly column for the Daily Mail.
"When I was growing up, I was always told to get touch-tight to my man in the box and to always be slightly in front of the striker so I could match his movement. It’s important McNair works on that."