Cassidy, making his first start back after a long term injury, left the field at 0-0 in this West of Scotland League Conference B clash, but by the end Rovers were on the end of a 4-1 defeat at John Cumming Stadium.
"Mark had a slight wee niggle,” Rovers boss Mark Weir told the Carluke and Lanark Gazette. “We took him off as a precautionary measure.
"It’s to do with his calves. He had said at half-time he was tightening up so he signalled five minutes into the second half that he needed to come off.
"The first half was really good but same scenario – not taking our chances.
"We could have been 2-0 up at half-time but we went in at 0-0 still delighted.
"But Mark coming off meant we had to make a couple of wee changes and I think it was a minute later, 1-0 down.
"There were plenty positives but it was silly goals lost again, mistakes cost us.
"When you make mistakes against Gartcairn, with the players they’ve got and the experience in their team, they punish you and basically that’s what happened.”
The game had started with Gartcairn missing an early first half chance, before Rovers’ Dillon Duddy and Dom Small both had shots saved.
James Frame and Duddy again missed further scoring opportunities and – after Cassidy's departure – Gartcairn scored two goals in 10 minutes via a tap in and a 25-yard free-kick.
Small shot home to pull one back for Rovers but the visitors added two more goals for a 4-1 win which flattered them.
"At 2-0 I was fearing the worst that it could be three, four or five nil,” Weir said.
"But at 2-1 I was thinking it was game on. The guys dug deep to score a goal and I thought we could have gone on and scored again.
"But unfortunately they went up the other end and before you knew it, it was another wee blip for ourselves and we were 3-1 down.”
Weir thinks skipper Cassidy will be fit for Rovers’ game this Saturday, a home tie against Clydebank in The Kilmarnock Pie West of Scotland League Cup first round, KO 1.30pm.
"Clydebank is a team that’s flying at the moment,” Weir said.
"Listen, as I said before the Auchinleck game a few months back, these are the games that you want to be involved in and play in.
"We will treat Clydebank the same as we treat all our games. We know we are in for a difficult task but if we can apply ourselves the way we applied ourselves on Saturday you never know, there might be a cup shock.”