Lallana has been out for a month since damaging his thigh on international duty with England and is back in full training.
The playmaker posted a picture of himself on Instagram, tying his bootlaces at Melwood with the message, “T minus 6 days and counting: Watford on my mind”.
Lallana’s return is a huge lift for Liverpool as they attempt to get their top-four bid back on track following Sunday’s home loss to Crystal Palace.
Lallana, 28, has contributed seven goals and seven assists in the Premier League this season and his return to action will take some of the weight of the shoulders of Philippe Coutinho and Roberto Firmino.
Liverpool boss Jurgen Klopp said: “Maybe Adam is coming back for the next game, of course it would help us in different situations, it’s good.
“We have now five or six days to train before Watford, we will use it, it will not be easy but possible. It’s enough.”