Welbeck suffered a serious hamstring injury early on against Tottenham while Holebas received his fifth yellow card, so must serve a one-match ban.
Defender Daryl Janmaat is expected to feature after being substituted at Spurs, while Sebastian Prodl, Etienne Capoue and Isaac Success are all back in first-team training. Captain Troy Deeney is recovering from knee surgery.
Bournemouth expect to have Colombia midfielder Jefferson Lerma available again following a hip problem suffered on international duty.
Wales midfielder David Brooks is stepping up his recovery from a pre-season ankle injury.
Watford provisional squad: Foster, Dawson, Mariappa, Prodl, Kiko, Foulquier, Cleverley, Doucoure, Hughes, Capoue, Deulofeu, Gomes, Janmaat, Cathcart, Kabasele, Chalobah, Sarr, Pereyra, Gray, Success.
Bournemouth provisional squad: Ramsdale, S Cook, Ake, A Smith, Rico, Lerma, L Cook, H Wilson, Fraser, King, C Wilson, Boruc, Francis, Kelly, Mepham, Surman, Billing, Solanke, Groeneveld.