The midfielder was back on the Liverpool bench after six weeks out with a thigh injury, but was called upon in the early stages at Vicarage Road when Philippe Coutinho limped off.
Lallana showed little sign of rustiness as his industrious running and smart use of the ball kept the Reds on the front foot en route to a crucial 1-0 victory in their battle for a top-four spot in the Premier League.
“It was a bonus. I didn’t expect to come on so early but these things happen,” the Englishman told Sky Sports after the final whistle.
“You’ve got to be prepared mentally for anything. Phil was unlucky to get a knock, sometimes when you get these knees to the side of the quad you can’t run them off.
“It’s a massive blow for us. Hopefully we can get him fit for the weekend because he’s such a big player for us.
“I had to come on and I felt I got into the game quite quickly, I didn’t feel like I’d lost too much rhythm. I was a bit tired at the end, but I’ve had five or six weeks out of the game so that’s only normal.”
On the significance of the result, which strengthened Liverpool’s third-place position in the table, Lallana said: “It was a massive win.
“Watford can be a tricky game. It was away from home and we played last of the weekend’s fixtures. It was a massive three points. Three games left for us now and I believe it’s in our hands. We just need to focus on Southampton at the weekend.”
The match was settled, of course, by Emre Can’s wonder goal.
In first-half added time, the No.23 darted towards the area to meet Lucas Leiva’s lofted pass forward and produced a moment of unbelievable magic.
Acrobatically contorting his body to connect with the ball, Can volleyed a finish high over Heurelho Gomes and into the top corner.
“I was right behind it and it was worthy of winning any game,” said Lallana.
“He was brave enough to try it – credit to him, it flew into the top corner and got us the win and, importantly, three points.”