The manager spoke to PA Sport about the England youngster and his club and international strike partner Wayne Rooney after their performances in Euro 2012.
Welbeck is tipped to score more regularly next term as he continues his development at the highest level while Rooney was a victim of circumstance in Ukraine and Poland as he looked short of fitness in the quarter-final against Italy.
"We are negotiating with Danny's brothers," stated Sir Alex. "They represent him. It's difficult to say where we are with that at this moment in time.
"If you look at the games he played for England at the Euros, he's never played three games in seven days before, well certainly not for us. So he did really well in that respect. That's where maturity and development comes in.
"We've fostered him. He's always had growth spurts and things like that, so we fostered him right that period when he was developing. I don't think he's got his full body yet. I think there's a lot of growth in him yet. I don't think he'll get taller, he's 6ft 3ins now.
"He's very powerful and, once that growth thing stops, I think you'll find he can play three games in seven games. There won't be a problem with that. He's got good movement, courage and confidence with the ball. Obviously, he will have to improve his goalscoring. I think he got nine goals last season but if you are going to be a top striker you have to get 20 goals or above. That will happen to him, I think he will do that."
Rooney's contribution at the Euros has been the subject of much debate in the press but there