The on-loan Rotherham winger made a spectacular early impression on the Motherwell fans with a 25-yard equaliser to salvage a 2-2 draw at Tannadice.
It was the 25-year-old's second appearance off the bench at his 13th professional club but it came as no surprise to McCall.
"He tore us to shreds when I was at Bradford years ago for Hereford," said McCall. "He has had loads of clubs but Lee Clark bought him for Huddersfield and Aidy Boothroyd bought him for Watford, so good managers have brought him to their clubs.
"He wasn't in the team at Rotherham, he injured his shoulder last season and couldn't get back in the team and they got promotion and the manager didn't play with wingers at times.
"So I knew him for a few years. I have never seen him do that right enough.
"He has got that in his locker, and he gives an outlet on the right with pace and he crosses a good ball as well."
Ainsworth's strike was not the only impressive goal of the game. Henri Anier gave Motherwell a deserved lead with a well-executed chest and volley in the 30th minute before United hit back with two goals in four second-half minutes.
Nadir Ciftci fired home in the 53rd minute and left-back Andrew Robertson ran unchallenged from well inside his own half to fire into the corner from 20 yards.
United boss Jackie McNamara hailed the impact made by 19-year-old Robertson, who turned professional in the summer following his move from Queen's Park.
"It's there for everyone to see, you see every United fan singing his name," said McNamara, who revealed winger Gary Mackay-Steven had failed a fitness test on a groin injury.
"He has been a revelation and he is getting better and better. I'm delighted for him because he has had a few wee chances in the last few games and that was a good goal for him."