The Spanish striker picked up his fifth booking of the season against Crystal Palace and is therefore suspended for the showdown at Stamford Bridge.
Costa has been in blistering form, helping the Blues to 11 straight top flight wins.
“It’s a big blow because he’s a top striker,” Courtois said.
“He scored his 13th goal of the season and he’s in top form.
“He’s been one booking away for many games now, so if he’d had a bit more luck he would have stayed like that until the New Year, when it’s gone, but hopefully Michy, or whoever the manager plays there, will do well.
“It’s a big blow because Diego Costa is a top striker”
Chelsea goalkeeper Thibaut Courtois
“For the manager, it’s not easy to choose players.
“With Cesc, Michy, Pedro, Ivanovic and Zouma we had a good bench and to be champions you need not only 11 players, but 18 or 22, the whole squad.
“We have a good team and we try to prove that every week.”
Courtois also opened up on how Antonio Conte has turned the club’s fortunes around since defeat to Arsenal in September.
“That defeat to Arsenal hurt us but we’ve come back stronger with the new tactics and system,” he added.
“It works well, everybody is doing their job and we keep on winning. The spirit is very important.
“This week was good because we played against smaller teams but they defend well and it’s hard to break them down.
“To win all three games 1-0 is very good and we’re happy. We now have two home games in front of our fans so we need to win them both.”