Welbeck led the line with an assurance beyond his years at the Emirates Stadium, causing the Gunners defence problems all afternoon with his intelligent movement before capping a lively performance with the crucial winning goal.
The 81st minute strike was Welbeck’s ninth in all competitions this season, and Smalling believes his team-mate’s rapid development in recent months is there for all to see.
"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. 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."
United’s win keeps them just three points behind bitter rivals Manchester City, who enjoyed a last-gasp 3-2 victory over Tottenham earlier on Sunday, and Smalling revealed Sir Alex Ferguson’s men remain confident of winning a record 20th league title.
"This was a massive win for us. Even at 1-1 we all knew we had to really win this game after what had happened earlier at City. I think we showed real character to do that," he added.
"We did see some of the City match on the way here and obviously we did know the result before we started.
"We knew they had scored in the dying minutes but we always know that we cannot rely on them to drop points.
"We always have to do our job and we did that. If we keep ticking off these wins then soon enough they will crumble.
"We just needed to make sure we won and not let that gap increase any more. It is still going to go down to the last few games."
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