The German tweaked his groin during Saturday's victory over Wolves and will miss the Blues' Champions League opener.
Meanwhile, N'Golo Kante and Callum Hudson-Odoi are edging closer to full fitness and could play some role against Los Che. Kepa, Tammy Abraham, Mason Mount and Fikayo Tomori are all set for their debuts among Europe's elite when they face Valencia.
It might be a step into the unknown for Chelsea's young team, but Kepa feels the "energy" and "enthusiasm" they demonstrated in winning 5-2 at Wolves over the weekend in the Premier League will serve them well.
The 24-year-old goalkeeper said: "Regardless of the fact we're a young team and it's only September and there is a long way to go, we are going to play with energy and try and attack the opposition.
"Many of the players in the team will be playing their first Champions League game, but we've been waiting for this for a long time. We spent the whole of last year trying to qualify for the Champions League, so we feel very enthusiastic.
"We can't wait to see the atmosphere at the Bridge and play in front of our fans in the most important European club competition."
Meanwhile, Valencia are in a state of disarray, with players up in arms about the sacking of manager Marcelino last week.
The squad are refusing to speak to the media - even though one player is required by UEFA to attend pre-Champions League press conferences.
Valencia have managed just one win in their first four La Liga games and currently find themselves down in 13th place in the table.