“It’s obviously disappointing because of the manner in which we lost, that’s the biggest thing,” Tomkins told West Ham’s official website.
“I think everyone wasn’t at the races like we normally are, so that’s the most disappointing thing."
“Everyone can probably hold their hands up and say they did not do as well as they could, so that’s the massive disappointment,” added the 26-year-old.
Orion Ighalo opened the scoring at Vicarage Road in the 39th minute after Nathan Ake dispossessed Andy Carroll in defence and crossed low.
The Nigerian doubled Watford's lead just after the break, firing home from Ikechi Anya's cut-back.
James Collins was dismissed for West Ham for a foul on Ighalo, as the visitors suffered a 2-0 defeat.
Over the course of the 90 minutes at Vicarage Road, Slaven Bilic’s side had 49% of the possession, took eight shots of which three were on target, and earned two corners.
West Ham are fifth in the Premier League table at the moment with 20 points from 11 matches, just a point behind fourth-place Manchester United.