The 19-year-old defender was thrown into first team action by Louis van Gaal after injuries to Chris Smalling, Phil Jones and Jonny Evans as well as a suspension for Tyler Blackett.
Alongside Marcos Rojo in the heart of the defence, McNair helped United to a 2-1 win at Old Trafford, with the youngster making a crucial headed clearance to deny Enner Valencia in the second half.
And United captain Rooney, who scored the opener but was later sent off for a wild kick at Stewart Downing, was impressed with McNair's display.
"That was probably one of the bigger tests a young defender can face, up against a team putting in lots of crosses into the box, but he took it and stood up to it, especially in the second half," he said.
"It won't get much tougher in terms of physical strength and aggression against him, but he dealt with it and we are all delighted with him.
"I saw the clearance he made at the end. I thought he was tremendous for us.
"Although we let Tom Cleverley go on loan [to Aston Villa] and Danny Welbeck go to Arsenal, we have brought in a young Manchester lad in Tyler Blackett who played every game until Saturday.
"The situation was forced on the manager but he has brought Paddy in and it's shown he isn't afraid to throw young players in."
Rooney's red card means he is suspended for the home clash with former club Everton on Sunday, but goalkeeper David de Gea believes United have the squad depth to deal with their skipper's absence.
"It's a very important game this Sunday, we got some very important points against West Ham with ten players in a difficult match," said the Spaniard.
"If we win against Everton we will be in a good position in the table. He's [Rooney] a really important player for us, but I think we have other players ready to play these games."
