Christian Norgaard fired the Bees into a 15th-minute lead and whilst Emmanuel Dennis levelled ten minutes after the interval, Brentford captain Jansson had the final say when heading home a 95th-minute winner.
Watford have now lost ten home games on the spin and Hodgson's team remain six points behind fourth-bottom Everton and now having played two games more.
The Vicarage Road outfit are 11 points behind fifth-bottom Leeds, both sides now having six games left.
The Hornets could yet get themselves up to 40 points but realistic Hodgson admitted that making such wild predictions would be ridiculed.
"One has to believe, one has to keep faith, one has to take some sort of heart from their desire and commitment and effort during the second half," said Hodgson at his post-match press conference, as quoted by Herts Live.
"And you just have to hope that six times 95 minutes more, they'll be able to do that.
"And maybe, who knows, we will pick up points when no one expected and we will get ourselves back in the race.
"But it's not the right moment in time to start predicting that from the next game onwards we are now going to sweep the board and kill all opposition that we find in front of us.
"Because if I say that, people will laugh at me."