Midfielder Tyler Fletcher was given his Scotland debut as a half-time substitute and Clarke confirmed the 19-year-old was one possible replacement for Gilmour.
The Manchester United player – son of former Scotland captain Darren – is one of four youngsters that have trained with the squad this week.
None of those are part of the squad for the World Cup matches against Haiti, Morocco and Brazil, and neither are Rangers’ Connor Barron, Andy Irving of Sparta Prague and Udinese’s Lennon Milller, who Clarke also named-checked.
“Those three are on standby and obviously Tyler joined us this week,” he said. “He’s trained well this week, so he’s a little bit closer than the other three, but I’d need to have a big discussion with my staff and decide the best way to go.”
Fletcher – whose twin Jack plays age-grade football for England – made his senior debut for his father’s former club in February and also came on in the final game of the season against Brighton.
Former Scotland winger Neil McCann put it to Clarke on BBC One Scotland that he looked “really accomplished” in a holding role.
Clarke responded: “Everybody was impressed – the players were impressed, the coaching staff were impressed, I had no doubts.
“I actually thought about putting him on as Billy came off but I just thought I’d have a little look, see how the game panned out before introducing him at half-time.”
Scotland midfielder Kenny McLean played with Darren Fletcher at the start of his own international career and said he “can’t speak highly enough” of Tyler.
“I said to the lads after his first session, I could see something special in him, so I was glad that he’d got on today. He deserved it after the week he’s had,” the Norwich City midfielder said.
“In my first session with Scotland, Darren stood out a mile. Fortunately for us, his son is pretty similar. There’s a lot of potential, it’s about using him the right way.”
