The 29-year-old joined United on a free transfer after his contract with relegated Newcastle United expired last month.
“I’ve been really pleased with the way he has trained,” Sir Alex told the Mail on Sunday.
“He has worked hard -but the main thing is he coming into work with a smile on his face.
“He is really enjoying himself.”
“There is no doubt he could really get his goalscoring form back on track with us,” he added.
“But really he has never lost it.
“Although he had a few injury problems at Newcastle, he kept up the excellent goals-per-game ratio he has had throughout his career.”