The centre-half netted twice in the second half at Ibrox to ensure the SPFL League One champions edged to a narrow 2-1 triumph.
The French-Tunisian's goals mean the Gers can still go the 2013/14 league campaign unbeaten - if they win their final two fixtures.
And legendary goalscorer McCoist admitted he was grateful to the larger-than-life personality for producing the goods once again when it mattered most. He said: "For long periods of the game we weren't great, particularly in the first half. You have to give credit to Ayr. I thought they played well and knocked the ball about well. That said, I thought we were disappointing, very disappointing, in the first half. We changed it in the second half and we were better.
"Not by a lot, but we created more. But we found a way to win a game again and that is a great habit to get into.
"That is really important. And in Mohsni we had a match-winner.
"He is now our joint-third top goalscorer with 12 goals to his name. It is a fantastic return for a centre-back.
"He is a box of tricks is our Bilel that is for sure! He has come up with some important goals for us and we have to be thankful for that.
"When we got him in from Southend we knew his goalscoring capabilities.
"He does get involved in stuff that he shouldn't.
"He is highly strung and we try to calm him down. But he is a real goal threat, particularly at set-plays."
McCoist revealed he had still not met with Rangers chief executive Graham Wallace to find out what the implications of the 120-day business review will be for him.
Asked if he had been told what his budget will be for next season, he said: "I haven't, not yet.
"But I don't think there is any doubt I will be seeing Graham before the end of the week."
Meanwhile, McCoist was scathing about the defending that lead to Ayr forward Alan Forrest, the brother of Celtic and Scotland winger James, netting an equaliser.
And he vowed to work hard on the training ground at Auchenhowie to ensure that there is no repeat of it either this season or in the future.
He said: "We lost a similar one up at Brechin at the start of the season. It was a schoolboy goal to lose and I was very, very disappointed with it.
"These mistakes are crucial at any level. The higher up the leagues you go in football the better players will punish you. But Ayr United certainly punished us.
"We have to eliminate silly errors like that. But, with the greatest of respect, it is not something we have been doing.
"It is just the 17th league goal we have lost this season.
"Yes, it is a mistake we will have to work on and make sure we don't do the same thing again. But it is not something we have done on numerous occasions this year."