McCall left his position on Sunday, with Motherwell having lost five league games in a row to slump towards the foot of the table.
However, the Fir Park club got back to winning ways on home soil on Friday, with Iain Vigurs the hero.
Without a win in their last five matches against United, who are now set to be overhauled at the top of the table over the weekend, Motherwell went into the match as big outsiders.
Yet Vigurs struck the winner from 20 yards in the 52nd minute, to ensure an impressive Motherwell performance gained reward.
United's Blair Spittal had the best chance of the first half but, sent through on goal, toe-poked wide from his one-on-one opportunity.
Motherwell were denied by the woodwork four minutes after the break, as Lionel Ainsworth fired an effort against the inside of the post from six yards after Craig Reid was denied.
The hosts did not have to wait long to take the lead, though, with Vigurs firing in the opener after being set up by Keith Lasley.
Ainsworth was denied Motherwell's second as he was adjudged offside, but one goal proved enough.
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