The popular Iceland striker has rejoined the Hornets for a third spell on loan from QPR and is likely to go straight into the starting line-up.
Teenage midfielder Ross Jenkins could also start after returning as a half-time substitute during the 5-0 FA Cup defeat at Chelsea.
Jenkins had been sidelined for two and a half months following a back operation.
Boss Malky Mackay is close to completing the signing of Scotland Under-21 midfielder Stephen McGinn from St Mirren, but he is not expected to link up with his new team-mates until Monday.
Slovenian defender Jure Travner has joined Norwegian side FK Haugesund on trial.