The 28-year-old sat out last Saturday's win over St Mirren and his absence will continue until Hibs get to the bottom of his problem.
"We are still without Scott Allan," head coach Jack Ross said.
"He has got a health issue that we are investigating further so he will be missing for the next upcoming period until we get him back fit and healthy.
"It's a private and confidential matter for him. If and when we have more understanding of it then I'd be prepared to comment on it fully, but for the moment it rules him out for Sunday and for the immediate games afterwards."
Hibs strengthened in midfield in midweek by bringing in 31-year-old Stephen McGinn, who helped Ross transform St Mirren from Championship strugglers to champions after joining the Buddies in 2017.
"He gives us leadership on the pitch in that area, and off the pitch," Ross said.
"We have it in the defensive part of the game quite obviously with some of the players we have in the squad. But in midfield we are relatively young.
"What Stephen has is proven quality and experience. He's a really good influence on the training pitch as well. He will play our part for us on the pitch this season.
"I'm delighted to have him here and I know he's excited by it as well. He is a good player who has had a good career. He has been in here since pre-season so that's good reward for him."
Hibs are aiming to be the first Scottish team to score against Rangers since Hamilton won at Ibrox just before the lockdown in March.
But they have a man in their ranks who played that day in midfielder Alex Gogic.
Gogic said: "Every game is different but to know that feeling is good. And you know that it's possible. If we take that into it, hopefully we can do it again.
"It's football, any team can do it on a good day. We just have to focus on what we do and hopefully we can get a win.
"We have been doing well as well so it's a good game for us."
Gogic returned to the side in Paisley after being forced to self-isolate and miss his debut Cyprus call-up following a false positive coronavirus test.
Gogic is now looking to ensure he continues to catch the eye of his national team by shining in the televised game.
"This is one of the biggest games for for us and to impress would be good, and hopefully get a call-up for the next time," he said.
"The most important thing was health-wise we were all good. It was a blow but it's OK, it wasn't meant to be.
"It was hard, you had to stay in the house. I had the spin bike and a programme so I kept on it but it's not the same as running. But I feel good, I feel fit.
"We were all frustrated because it was an important game for us and I wasn't able to play after two negatives.
"But now you can see it more clearly, it's the safest for everyone else to be honest."