The Croatia midfielder picked up an Achilles problem in Wednesday's 3-2 loss at West Ham, and could now miss the next two matches.
Defender Fikayo Tomori is still sidelined with a muscle problem but is edging closer towards a return to training.
The trio were forced to miss last week's defeat at Southampton after Gray reportedly flouted social distancing rules by hosting a birthday party.
Chelsea provisional squad: Arrizabalaga, Azpilicueta, Christensen, Rudiger, Alonso, Kante, Barkley, Jorginho, Pulisic, Willian, Abraham, Caballero, Zouma, James, Emerson, Loftus-Cheek, Gilmour, Mount, Hudson-Odoi, Giroud.
Watford provisional squad: Foster, Gomes, Masina, Femenia, Holebas, Mariappa, Cathcart, Kabasele, Cleverley, Chalobah, Hughes, Doucoure, Capoue, Pereyra, Sarr, Deeney, Welbeck, Pedro, Gray, Quina.