Christian Pulisic will not return for Chelsea until after the international break as he continues to struggle with a hamstring problem.
Frank Lampard revealed in midweek that a scan had only shown "very minor" damage for the United States international, who sustained the injury in the warm-up ahead of last weekend's 3-0 win over Burnley.
The Chelsea boss was initially positive about Pulisic's recovery but confirmed on Friday he had stopped training this week due to ongoing discomfort.
Pulisic will consequently be unavailable for Saturday's meeting with Sheffield United in the Premier League at Stamford Bridge, and his absence means Chelsea will be without one of their most potent attacking weapons.
Since football returned in June, Pulisic has attempted more shots (28) and completed more dribbles (19) than any other Chelsea player in the top flight. He has also been involved in a team-high six goals (four scored, two assisted) in the competition, level with Olivier Giroud and Willian.
"Pulisic is not fit to play. He's still suffering with his hamstring. He's not [training] at the moment," said Lampard.
"To be clear, he trained at a low level in midweek when I made those quotes and he wasn't comfortable, so we have to keep looking for the right solution."
Lampard confirmed that Kepa Arrizabalaga will be in the squad this weekend as he continues to recover from a shoulder problem.
"He [Kepa] has recovered, he's still getting a slight bit of pain but he's in the squad for tomorrow," said the Blues boss.
Timo Werner replaced Pulisic in the starting line-up for the Burnley game and the Germany international has played more minutes for Chelsea this season than any other player.
Lampard hopes to be able to grant Werner – who has been involved in four goals in his past three Premier League games – a rest soon and would not clarify if that will come against the Blades.
"I'm not going to give the team away, but I think it was clear with the Burnley game I was planning on giving him a rest. It was more because I had options and the journey to Russia and back," said Lampard.
"Timo's played a lot of games, but at the beginning there's always a balance in my job of players that are playing well and enjoying it, finding their way in the team and making relationships and feeling the league if they're new to the league, and I think that's been a big deal for Timo.
"I've been happy with that point. As we look forward, I will try to manage those minutes to keep him as fresh as I can, but at the minute he's going well, so we'll see."