The midfielder was ruled out for at least "a couple of months" by manager Jürgen Klopp last week and will miss out on the trip to the Vicarage Road for the 2017-18 curtain-raiser this weekend.
Taking to Instagram, Lallana insisted that he is "gutted" to "be missing the start of the season" but added: "I have nothing but confidence in how the team is going to do."
He noted how Liverpool's first-team squad "are in great shape and ready to attack it" and explained that he is focused on "getting back and supporting the boys" as he thanked fans for "all the kind messages" of support.
Liverpool will "wait and see" on Lallana's return date
In addition to Liverpool's opener against Watford, the England international will miss the club's two-legged Champions League play-off qualifier against Hoffenheim as well as huge league clashes with Arsenal and Manchester City.
Speaking on Friday, Klopp confirmed that the news of Lallana's setback is not news that the club "would have wanted" and explained: "Unfortunately, the damage is such that it will likely mean Adam is out for a couple of months not weeks, so I think we can pretty much rule him out of August and September."
The German declared that the club's medical staff will "monitor and wait and see" beyond those two first months, adding that Lallana "is a boy with a world-class attitude to everything professionally."
Klopp also praised him for "always" returning "in good time from injury setbacks" since the manager had arrived at the club, declaring that they "will see" as to when he returns.
Klopp's other options include Georginio Wijnaldum and James Milner, who he described as "a new midfielder" after playing centrally in their final pre-season friendly against Athletic Bilbao in Dublin on Saturday.
Otherwise, Ovie Ejaria and Ben Woodburn represent youthful back-up alternatives - the pair likely to make their first appearances of the season when Liverpool start their League Cup campaign in mid-September, barring more injuries.