Wales international Mepham is thought to have suffered ligament damage during the FA Cup third-round win over Luton.
Norway forward Joshua King (hamstring) and Jack Stacey (thigh) have added to a packed treatement room – with Nathan Ake (hamstring), Adam Smith (foot), Lloyd Kelly (thigh), David Brooks (ankle), Arnaut Groeneveld (foot) and Charlie Daniels (knee) also continuing their own recovery.
Chalobah was withdrawn at half-time during last Saturday's 3-3 draw FA Cup draw against Tranmere, but has trained this week.
Craig Cathcart is also set to feature, but Kiko Femenia (hamstring), Christian Kabasele (suspended), Sebastian Prodl (shin), Daryl Janmaat (knee), Danny Welbeck (hamstring) and Tom Cleverley (foot) are all sidelined
Bournemouth provisional squad: Ramsdale, Francis, S Cook, Rico, Simpson, Gosling, Lerma, L Cook, H Wilson, Solanke, C Wilson, Boruc, Jordan, Surman, Stanislas, Fraser, Billing, Dobre, Surridge.
Watford provisional squad: Foster, Cathcart, Hughes, Doucoure, Capoue, Deulofeu, Deeney, Sarr, Gomes, Dawson, Masina, Foulquier, Chalobah, Pereyra, Quina, Gray, Success.