Costa was charged with violent conduct by the Football Association for an apparent stamp on Emre Can in Chelsea's League Cup semi-final win over Liverpool on Tuesday – a game Fabregas limped out of with a hamstring injury.
Chelsea's top scorer will discover if he has received a ban on the eve of the game, while the league's most prolific assist provider's fitness will be tested.
If unable to call upon the Spain duo, Mourinho will have to reshuffle his pack – which may have been on the cards anyway having been taken to extra time by Liverpool – though any hints will not be forthcoming after the Portuguese cancelled his pre-match media conference, having been fined €33,000 this week over comments about referees.
Chelsea sit five points above City at the Premier League summit, with the champions having drawn at Everton and lost to Arsenal after drawing level with Mourinho's side with a strong run of festive results.
The late kick-off at Stamford Bridge provides City's first chance to respond to last weekend's FA Cup elimination at the hands of Championship promotion hopefuls Middlesbrough, with Chelsea themselves still rebuilding confidence after a stunning 4-2 loss to Bradford City.
Manuel Pellegrini has his own star striker, Sergio Aguero, fit and firing after a knee injury, though Frank Lampard's return to Stamford Bridge is sure to intrigue.
Chelsea's all-time leading goalscorer joined City in pre-season in a move that has caused some controversy and he has already come back to haunt the fans who revered him so much – hitting his new employers' equaliser in September's 1-1 draw between the sides at the Etihad Stadium.
Lampard was regardless handed a warm reception by Chelsea's fans that day, with another generous welcome expected upon his return to west London.
Any hopes Pellegrini may have held of having Yaya Toure back in his midfield were dashed on Wednesday by Ivory Coast's progression in the Africa Cup of Nations, while Samir Nasri remains sidelined by a calf injury, meaning Lampard could be in the team from the start.
Mourinho has further concerns in his defence with Filipe Luis (calf) and match-winner Branislav Ivanovic (foot) also sustaining injuries in midweek.
