The midfielder replaced Jordan Henderson in the 71st minute at Old Trafford with the Reds 1-0 down after Marcus Rashford's first-half finish.
And the 31-year-old was perfectly positioned to convert Andy Robertson's cross five minutes from time to restore parity and maintain Liverpool's unbeaten start to the Premier League campaign.
Asked what his instructions were from the manager, Lallana told Sky Sports: "Just to make an impact, there were 25 minutes left.
“We probably weren't at our best today but sometimes you have to look to the bench and you have to come on and change the game.