The England under-21 striker, who misses the Black Cats’ Boxing Day trip to Old Trafford through ineligibility, has shone for Steve Bruce’s side during the first half of the campaign.
Welbeck has bagged five goals in his last six games, and has looked increasingly at home in the Premier League, operating in both wide and central roles to great effect.
“Exactly the reason I put Danny on loan was to play regular football,” Sir Alex told his pre-match press conference. “In the last couple of years around the first team, he’s never played more than a couple of games in a row. Not since his youth team days has he played more than four games in a row.
“One of Danny’s problems in the last couple of years was that he was growing so big and quickly, he was weak in certain parts of his legs, particularly the quadriceps and knees, and this caused a lot of problems for the boy, strength-wise.
“You look at him now and you see the growth, his body has now filled out. He’s a big lad, he’s powerful, he’s handling the physical part quite easily now. I think he’s been one of Sunderland’s best players.”
Welbeck is due to return to United at the end of June, although Steve Bruce has already voiced an interest in signing the striker on a permanent basis.