Crerand, a member of the club's 1968 European Cup-winning side, believes Mata's arrival can give David Moyes' side the lift they need.
"I think any signing, particularly a player of Mata's quality, will lift everyone," said Crerand. "The crowd need a lift, because they're a bit disappointed. The crowd and everyone at the club just needs a lift right now, and maybe the signing of Mata can do that. He's that kind of player.
"Chelsea are strong in the middle of the park and have three or four world-class players there, just like United were a few years ago. But when Wayne Rooney and Robin van Persie are back, that will also make a big difference to United, because they're two top players."
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