The pair missed the Scottish Cup defeat to Aberdeen on Sunday through injury, with midfielder Isaac Osbourne dropping back to assume a place on the right side of the Jags rearguard.
Thistle head to Easter Road aiming to halt their recent poor run and get back to winning ways as they bid to climb the SPFL Premiership table.
Boss Alan Archibald will have Aaron Muirhead available for selection again following his suspension while Conrad Balatoni will miss out after being sent off at Tannadice a fortnight ago.
"Stephen's suspension was up and he was available for the Aberdeen game, but he wasn't fit, he has got a hip problem," Archibald said.
"He went to see a specialist on Monday so we will have to see how he is over the next few days. Jordan never trained on Monday and he missed most of last week after suffering a back spasm.
"Stephen has had a hip problem for a while to be honest. He has not been playing through the injury, but players always have wee niggles here and there. When he got the suspension, we got him in for a scan and something showed up.
"He saw the specialist on Monday. It is a worry with the two of them being out at the same time."
If O'Donnell and McMillan don't make the squad for the Hibs clash, Osbourne is likely to be given the nod to start at right-back once again.
But Archibald has no fears about asking the former Aberdeen star to fill in following another impressive performance on Sunday.
He said: "Ozzy is just one of those guys that gives you his all every day and in every game.
"You can hang your hat on him, you know he will give you his all and put in a real shift for the team no matter where you ask him to go and play."
It is not just at the back where Archibald has problems to solve, with Thistle again failing to score as they crashed out of the cup.
On-loan Liverpool youngster Henoc Mukendi made a rare appearance on the Jags bench against the Dons after failing to make the breakthrough since his summer switch across the border.
Archibald said: "We feel there are a few guys ahead of him at the moment. He was on the bench on Sunday and did well in training last week."