He told the Daily Mail: "Mata reminds me of Dimitar Berbatov. He was brilliant for Tottenham and he looked every inch a Manchester United player before he signed.
"But White Hart Lane has a smaller pitch than Old Trafford and Berbatov didn't have the speed to get around it. Similarly, Mata stood out at Stamford Bridge, which is also tight.
"There is no doubt he has talent but does Mata have the legs for Old Trafford's wide open spaces?"
United are currently seventh in the Premier League.