Birtles believes the goals will come as soon as he gets regular action up front.
"The difficulty for him is he's not getting a clear run in the team," Birtles told the Manchester Evening News. "He's playing sporadically.
"He's out wide sometimes, deep sometimes. Strikers can't do that. It's unfair.
"As a striker you need to have four or five games where you can get continuity and an understanding with somebody who you are playing with up front or in the hole behind you.
"You need to be 100 per cent sharp - more so than in any other position on the pitch. I'm a fan. I wasn't sure about him at first when he was a bit green. I didn't know if he was going to be one to make it.
"Welbeck has come on and is maturing unbelievably well. His awareness has become one of his better points.
"Before I didn't think he made the right decisions when he got into the area. Maybe he was a bit too selfish. Now he is picking players out if he sees an opportunity that's better for somebody else. Before he would have gone himself.
"That's an important learning curve for a young player and he couldn't be at a better place than United to do that. He's got pace and he's got intelligence - and I think with people like Ryan Giggs around him, they will tell him things and he will listen.
"As you get older you tend to take things on board a little bit more. I think he's going to do that."