Welbeck had a loan spell at Sunderland last season and Smalling says that the 21 year old has improved his game so much, truly benefitting from the experience.
"Danny has a knack of scoring good goals this year for us and at vital times. That's a great asset for us to have," Smalling told the Manchester Evening News.
"This year he is getting his chance and he is such a threat. He causes defences so many problems and he did that again against Arsenal's. It is great for us.
"He has come on leaps and bounds. He has always had his pace and ability to finish, but he is so much stronger these days.
"Sometimes he has to play up front on his own with Wayne Rooney just off him at times and when you have that responsibility you cannot afford to lose the ball when you are in that position.
"You have to be strong to deal with that and that is something his game has gained and he is scoring goals as well. He is a proper Premier League striker now.
"He has come in and joined the show and if he keeps this form up to the end of the season he is going to be great for us and he'll do well for United and, hopefully, for England in the summer."