Liverpool brushed aside Middlesbrough to get back to winning ways and leapfrog Arsenal into 2nd place in the Premier League.
The reds reached 40 Premier League goals for the season in Matchday 16.
The pre-match focus was on the goalkeeping situation and Klopp opted to take Karius out of the firing line and bring Mignolet back in.
Boro, with just 13 goals to their name this season, provided little threat although Mignolet dealt with any danger comfortably.
At the other end, Lallana scored the first and third, and he provided the final touch to Divock Origi who scored his fifth goal in five games.
Rumoured before kick off - Loris Karius dropped to the bench with Simon Mignolet returning to the first team. Klopp said before the game that Mignolet was performing well in training so there was no need to keep Karius under pressure in the reds goal.
The surprising team news was that Joel Matip was a late withdrawal. An ongoing issue with his ankle - the third period out for a player who had surgery last summer. That could spell problems going forward for the imposing central defender.
But Ragnar Klavan was chosen ahead of Lucas Leiva to deputise and it added some solidity.
Although Middlesbrough are currently ranked 91 out of 92 professional English teams based on the number of goals scored this season.
The first half started evenly - Liverpool with a couple of half chances - Middlesbrough seeking to utilise the wings.
But by the midway point the reds moved into the ascendency.
Nathaniel Clyne powering down the right wing, sending in a powerful cross, met by the force of Adam Lallana's head and it was 1-0.
Boro looked to fight back - enjoying a brief spell of possession but the reds dealt with a flurry of corners and free kicks.
Towards the end of the half Sadio Mane saw a shot rebound off the post and Origi latched on to the rebound but it went narrowly wide.
Second half and the reds came out in a determined mood.
Firmino headed wide, Mane could have had two goals.
But it was a wonderful team move, as the clock struck 60 minutes, that put the reds ahead. Good build up by Mane, who found Lallana, who stroked the ball into the path of Divock Origi who scored his fifth goal in five games.
Daniel Sturridge's latest injury has been seized upon and Origi has surely overtaken him now in the Anfield pecking order.
And Origi was on hand to play the ball across the box for a simple Adam Lallana tap in less than 10 minutes later.
3-0. A comfortable win and the 38th, 39th and 40th goals of Liverpool's league campaign.
The reds move in to 2nd although they now enjoy a weekend off before a trip to local rivals Everton on Monday night in the pre-Christmas treat.
Man of the Match: Adam Lallana