KEEN CONTEST: Marcus Rashford (centre) scores a goal past Liverpool's goalkeeper Alisson Becker (left) during the Premier League match.
Marcus Rashford's controversial goal handed Ole Gunnar Solskjaer's struggling Red Devils a 1-0 half-time lead, but they could not hold out for a much-needed victory as they left substitute Lallana unmarked at the back post to level with five minutes to play.
Liverpool move six points clear of Manchester City at the top of the table, but will still feel this was a missed opportunity to inflict more pain on a United side sitting 13th in the Premier League, just two points off the relegation zone.
The two most successful sides in English football could not have come into the game in more contrasting form.
However, United answered Solskjaer's calls to rise to the occasion.
Liverpool were without Mohamed Salah through injury and the Egyptian's pace and trickery was badly missed.
The visitors were able to welcome back goalkeeper Alisson Becker for the first time in two months after a calf injury, but the Brazilian was rarely tested as United were happy to sit back and counter-attack.
Solskjaer changed his formation to a 3-5-2 in an attempt to limit Liverpool's flying full-backs Trent Alexander-Arnold and Andy Robertson and it was largely effective as the hosts restricted the league leaders to precious few chances.
Liverpool's best opening before the break came when Sadio Mane finally found some space to counter-attack, but from his cut-back Roberto Firmino fired too close to David de Gea.
United had not scored in their two previous matches prior to the international break, but finally produced a moment of quality going forward to take the lead, even if fortune was also on their side for the goal to stand.
Victor Lindelof looked to have upended Divock Origi on halfway, but no foul was given by Martin Atkinson and as United broke forward, Daniel James's cross perfectly picked out Rashford to prod past Alisson.
Liverpool manager Jurgen Klopp was furious on the touchline even before a VAR review failed to overturn the decision.
The visitors' frustration only grew moments later when Mane outmuscled Lindelof to turn in what he thought was an equaliser, only for VAR to this time rule the goal out for a handball by the Senegalese.
Klopp sprinted down the touchline at the half-time whistle, but was unable to change the pattern of the game after the interval as Liverpool continued to dominate possession without creating many chances.
Indeed United could have doubled their advantage when Rashford and Fred fired shots just wide of Alisson's goal on the counter-attack.
Lallana had played just 10 minutes in the Premier League this season prior to coming off the bench 20 minutes from time.
But he was in the right place at the right time to make a telling contribution in the title race as he tapped home at the back post from Andy Robertson's cross.
United were than camped inside their own box for five minutes of stoppage time.
Alex Oxlaide-Chamberlain was inches away from a winner as his driven shot flew just wide and United had another VAR review go their way deep into added time when Fred was adjudged to have played the ball with his shoulder rather than an arm inside the area.