Gleeson was forced off after bowling the second ball of his fifth over during the Royal London One-Day Cup quarter-final defeat to Surrey on Thursday night.
It was later revealed that the 28-year-old, who has one of the best economy rates in the 20-over competition this season, looked to have sustained a side strain.
And Ripley said: “It doesn’t look very good at all so unless a miracle happens overnight, it looks like he’s going to be struggling for Saturday.”
Alex Wakely bowled the remainder of Gleeson’s over, but conceded 24 runs, which eventually proved costly as Surrey squeezed home on the final ball to win the Royal London One-Day Cup quarter-final by one wicket at the County Ground.
And when asked about Wakely’s decision to bowl rather than hand duties over to the likes of Josh Cobb or Rob Keogh, Ripley said: “Cobby strode in to bowl so I don’t quite know what happened, but no one will be feeling it more than Alex.
“We had plenty of bowling in the bank and obviously we didn’t expect the outcome that we got.
“I’m sure Alex will be disappointed with the balls he bowled, but his captaincy was excellent throughout.”
Northants had showed great character to get back in the game, but were left heartbroken by the agonising late defeat.
They must now switch their attentions to the 20-over semi-final against Nottinghamshire at Edgbaston.
And it looks likely that they will have to dig deep in their bowling reserves, with Gleeson and Seekkuge Prasanna, who is also set to be unavailable, missing.
“We’ll dust ourselves off, get on the bus in the morning and, no matter what 11 we put out, it’s another great opportunity for us in our strongest format to have another great day.
“Once we get there to Edgbaston on Saturday morning, we’ll be ready to go.”