The France midfielder suffered a torn hamstring on international duty and is training alone while he recovers.
The Chelsea manager said: “Everybody was in training today, we had a fantastic training session, high quality, good atmosphere. Except for N’Golo Kante, who did individual work.
“It’s very likely Porto is too soon. He is doing individual training and from then on you can do the maths.
“Tomorrow he will miss the game and can’t be in the squad. Then we arrive on Sunday, Monday and Tuesday to prepare for the game, so that leaves us with only three days.
“He has a little muscle injury, as was communicated from the national team. So my realistic approach is to think after Porto and to think about the second game against Porto.”
Tuchel has confirmed Timo Werner will start against West Brom, despite the striker’s ongoing problems in front of goal after missing a sitter for Germany in this week’s defeat to North Macedonia.
Chelsea predicted XI: (3-4-2-1) Mendy; Azpilicueta, Thiago, Rudiger; James, Jorginho, Kovacic, Alonso; Havertz, Mount; Werner