The 25-year-old marked his second appearance off the bench at his 13th professional club by firing into the top corner in the 76th minute to earn Motherwell a 2-2 Scottish Premiership draw at Tannadice.
McCall said: "He tore us to shreds when I was at Bradford years ago for Hereford. 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.
"No, 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 after Motherwell were caught over-extending themselves on the break 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 games that he has put past and that was a good goal for him."
When asked if he was surprised by the teenager's progress, McNamara said: "I am in a certain way but once you work with the lad and see how focused he is, he has a steely determination that he has shown from day one. He is a very level-headed and intelligent lad."
McCall revealed his immediate emotion was disappointment that his team had not built on a dominant first-half display.
He added: "There were four great goals, but the two goals we lost were so disappointing. The second one we let the kid run 50 yards. In fairness to him it's a good finish but we have got to close the ball.
"I'm disappointed with the way we let them get back into it, although they deserved it with the way they played. We certainly deserved the equaliser and then both teams just went for it."
Motherwell played without a wide-right midfielder before Ainsworth came on, a formation that saw James McFadden dropped.
McCall said: "(Ryan) Gauld needed looking after in the pockets so we had to play one extra in the middle of the park. But I just don't like playing one up on their own. We had to go lopsided and play without a natural right-sider.
"We knew the left-back would be an outlet, hence my anger at letting him come so far down the park because we had been warned about that.
"I have left Zaine Francis-Angol out twice now after two good performances and I thought he was our best player against St Mirren and deserved to keep his place. It was nothing detrimental against James."