The Serbian called time on his Chelsea career this summer and completed a surprise £40million switch to Old Trafford.
Jose Mourinho had been looking for a midfielder and Matic was keen on a reunion with the Portuguese boss.
However, Matic’s move to United still raised a few eyebrows with Chelsea agreeing to sell to one of their title rivals, strengthening the Red Devils and improving a position they struggled with last term.
Matic earned plenty of praise for his display in United’s 4-0 win over West Ham yesterday as fellow debutant Romelu Lukaku scored twice – and former Arsenal defender Keown is a big fan of the 29-year-old.
“Romelu Lukaku will grab the headlines with his two goals but we must not overlook the importance of Nemanja Matic to Manchester United’s victory,” he told the Daily Mail.
“That was the best midfield performance of the Premier League’s opening weekend — has there been a better debut from a left-footed midfield player at Old Trafford since Bryan Robson in 1981?
“He oozed class — assured on the ball, calm under pressure. He kept feeding the quality players around him, taking 89 touches, the most of any United player.
“Out of possession, he positioned himself in front of the back four, allowing Paul Pogba to roam forward.
“At West Ham goal-kicks he saved the centre backs from their defensive duties, parking himself in front of the target men and winning the aerial battles.
“Matic is a magnet to danger but is quick to return to his position. Watching him is like watching a tennis player returning to the centre of the court after each shot.
“It is astonishing that Chelsea let him go. As he showed on Saturday, he has strengthened their rivals.”
Matic will be looking to make it two wins from two for United when they travel to Swansea on Saturday (12.30pm).