Morrison, once of Manchester United, was best afield in last night's Capital One Cup victory over Cheltenham.
"As a young kid coming from Manchester thinking he could walk into the team, it was a bit of a shock to him that he didn't," said McDonald.
"I think the penny has dropped now and he's come back a wiser young man and we look forward to him keep knocking on the manager's door.
"Now he has to keep scoring, he has to keep the ball, his all-round performance has to be good and he has to be consistent."
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