Young defender Cathcart has joined the Hornets from Manchester United and Iceland striker Helguson is back at the club where he scored 64 goals in five years after re-signing on loan from QPR.
Skipper Jay DeMerit should also be available after missing the last two games with the groin injury he suffered on international duty with the USA.
Boss Malky Mackay will therefore be faced with a selection dilemma, having named the same side in successive 1-0 wins over Barnsley and Plymouth which lifted Watford to seventh.
Teenage defender Dale Bennett is likely to step aside for DeMerit despite impressing on his first two senior starts.
Midfielder Scott Severin faces a couple more weeks on the sidelines with a groin injury.
Meanwhile, Wayne Brown is Leicester's only injury concern.
The defender hurt his shoulder in conceding the penalty which Peterborough scored in the 1-1 draw at the Walkers Stadium.
Brown was substituted at half-time in that game and remains a doubt. He will be assessed closer to kick-off with Bulgaria international Aleksandar Tunchev on stand-by to make only his second league start since January.
On-loan Sunderland striker Martyn Waghorn is pushing for a recall. Dany N'Guessan would be most likely to make way after he was substituted at half-time against Posh in a tactical switch by boss Nigel Pearson.
On the game at Vicarage Road, Pearson said: "It will be tough. We have played pretty well so far this season and I expect us to go there and do the same again. Whether we win it or not is another thing, but we will go and try to win the game.
"We will treat it like every other game. We will do our preparation on the opposition but, ultimately, it is about what we do ourselves. It is about getting the right team and making sure we play to our maximum."
Sporting Life