The former RB Leipzig attacker had only just shaken off his severe knee injury before damaging his hip in training, and Blues fans are anxiously awaiting updates on his condition after he sat out Saturday's 4-0 FA Cup third-round win over Preston North End.
Ben Chilwell (hamstring), Carney Chukwuemeka (knee) and Benoit Badiashile (unspecified) are also on the touch-and-go list as they enter the final stages of their recoveries, but all of Reece James (hamstring), Wesley Fofana (knee), Marc Cucurella (ankle), Robert Sanchez (knee), Lesley Ugochukwu (hamstring), Trevoh Chalobah (thigh) and Romeo Lavia (thigh) are unavailable.
In addition, Nicolas Jackson is away with Senegal on Africa Cup of Nations duty, while Andrey Santos - back from his unsuccessful loan spell at Nottingham Forest - is cup-tied.
With Jackson absent and Nkunku doubtful, Armando Broja - who opened the scoring against Preston - is seemingly guaranteed to slot into the number nine role, likely with Raheem Sterling and Cole Palmer reprising their roles behind him.
Given the tight turnaround, though, Mykhaylo Mudryk could cede his place to Noni Madueke, while Moises Caicedo played the full 90 against Preston and is also at risk of dropping out for the fresh legs of Conor Gallagher.
Alfie Gilchrist went off to a standing ovation on Saturday evening, but he will surely be a candidate for the axe on Tuesday, as a man 19 years his senior in Thiago Silva seeks an immediate return to the backline.
An expected Gilchrist-Silva swap should see Malo Gusto revert to the right as Levi Colwill is shifted out to left-back, allowing Silva and Axel Disasi to form Dorde Petrovic's central barrier.
Chelsea possible starting lineup: Petrovic; Gusto, Disasi, Silva, Colwill; Gallagher, Fernandez; Madueke, Palmer, Sterling; Broja