The Spain international was due to return to England on Friday to have an afternoon training session with the mask on in preparation for the Newcastle match.
Interim manager Guus Hiddink is already without Radamel Falcao and on-loan Alexandre Pato, while Loic Remy has been missing with a calf injury, so he cannot afford to lose Costa as well.
Although Hiddink did not reveal the identity of the player involved in the clash with Costa, Chelsea later confirmed it was Tomori.
Speaking at his Friday press conference, Hiddink said: “Yesterday, in training, Diego got in a duel and had his nose broken. It was one of the youngsters of the academy, who we like to participate.
“It was not a tough duel, but he put his nose on the back of the kid's head. He was very unlucky and his nose was broken. He will return today. They'll try and give him a mask as well - we are a team with a lot of masks.
"A Zorro team - Azpi, Matic, now Diego. He will come back and do some light training in the afternoon because he must experience how he deals with the mask. Then he will join us in the hotel later on.
“It was just a small incident, just a duel. But it's very fragile when you hit your nose on the back of the head of a firm young man. I think Diego will be available. We'll see how he reacts this afternoon in a very light session, but I hope – I'm not sure – that he will be available.”
- Telegraph