Abdoulaye Doucoure will not be available for Saturday's visit of Nottingham Forest, with manager Frank Lampard revealing the midfielder is set to miss around three weeks with a hamstring injury.
The Mali midfielder limped off during the first half of last weekend's clash with Aston Villa and has started his rehabilitation at Finch Farm this week.
Amadou Onana, who stepped off the bench as a late substitute at Villa Park, is in contention to make his first start for the Blues, with Lampard explaining he will give himself "as much time as I possibly can" as the midfielder continues to build his fitness.
Conor Coady, who started the previous match, could also make his home debut.
Allan – described as Lampard as "a really good pro" – is competing for a starting berth after missing a large chunk of pre-season through injury.
The manager also revealed Dominic Calvert-Lewin is "back out on the grass" as he continues his recovery from a knee injury sustained in the week leading into the beginning of the 2022/23 Premier League season.
Yerry Mina (ankle), Ben Godfrey (fractured fibula), André Gomes and Andros Townsend (ACL) all remain sidelined.
Speaking at his pre-match press conference, Lampard said: "Allan is fit, he is in the squad. He's back. He missed games in pre-season and, obviously, he's missed that part so we're trying to get him match-fit but he's a really good pro and he's fit, ready to go.
"Doucoure is probably going to be about three weeks from now.
"[Onana] is certainly in contention. That's a call for me to make and I'm giving it as long as I possibly can until the game tomorrow because as much as we want his impact - and I think he's going to have a big impact, as we saw a glimpse of that last weekend - we need to make sure we do the right thing by the player and the team.
"He missed big chunks of pre-season and this is the Premier League. I don't want to throw him in there too soon - I want to use him at the right time and give him the right minutes, so that's something we'll be managing over the next couple of weeks. We don't want an injury situation either, so it's important to get that one right for the longer term picture."