Skipper Chris Morgan is currently sidelined with a hamstring strain and Kyle Bartley, on loan from Arsenal, suffered a depressed fracture of the cheekbone in the midweek draw at Nottingham Forest.
Matt Lowton, 21, replaced Bartley on Tuesday night and Speed also has the option of playing Johnny Ertl in central defence alongside Nyron Nosworthy or another recent loan signing, Ritchie De Laet.
Manchester United defender De Laet made his first start against Forest at full-back. French defender Jean Calve, rested in midweek, could return in that position if required.
Speed, also without suspended striker Jamie Ward, revealed he has doubts about several other unnamed players for the Watford game, while long-term absentees Darius Henderson, Ryan France and Lee Williamson are continuing with their rehabilitation.
Watford boss Malky Mackay has a fully-fit squad at his disposal for the trip.
Mackay saw former Hornets boss Brendan Rodgers come out on top in midweek as Swansea won 3-2 at Vicarage Road but could name an unchanged side against the Blades.
Watford are unbeaten away from home and striker Troy Deeney is one man hopeful of pushing into the starting XI after he came off the bench to score against the Swans.
Andrew Taylor returned in midweek after being ineligible to face parent club Middlesbrough last weekend while midfielder Stephen McGinn is another option if Mackay wants to make changes.
Strike pairing Danny Graham and Marvin Sordell have scored 13 goals between them this season and both have boosted the Hornets this week by signing new contracts.
Young defender Dale Bennett has recovered from an ankle injury and will join up with the squad.
Source: DSG
