The midfielder sprinted in on goal but Kovacevic's well-timed tackle denied St Mirren the opening goal.
But Michael Kovacevic's perfectly-timed challenge stole the ball from his feet as he was about to pull the trigger.
Kovacevic was injured whilst tackling Fleck in first-half injury time and the full-back was replaced by Keith Watson before the interval.
Dundee United manager Peter Houston was again without defender Andy Webster through injury, with Sean Dillon handed a start in place of Michael Kovacevic.
Fleck took the penalty he won after a nudge in the back by Mihael Kovacevic as the forward thrust into the United box after 78 minutes.
Mihael Kovacevic failed to deal with a Jody Morris cross, allowing Moon headed the ball into MacDonald's path and he rifled a volley past Dusan Pernis.
But his season was cut short in Italy and after initial placenta fluid treatment with specialist Mariana Kovacevic, he went on to have ankle surgery in Amsterdam.
Prince Buaben returned to add experience to midfield in place of Stuart Armstrong, while Mihael Kovacevic was on the bench for the first time since being injured during pre-season.
Maja Kovacevic Tomic, the court's spokesman, is even happier.
Boyd almost doubled Rangers' advantage seven minutes before half-time when he turned in the box and sent a powerful drive towards goal but Mihael Kovacevic got back to make a crucial goal-line clearance.
Falkirk next claimed for a penalty after a Brian McLean shot struck the arm of Mihael Kovacevic in the box, but referee Calum Murray was unmoved as manager Steven Pressley fumed on the touchline.
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