With Manchester City romping away with the title race, United had to win at the Hawthorns to have any chance of reeling in their local rivals.
After banging in a breathtaking 32 goals in the first nine games of now 12 match unbeaten run, the Reds had hit a bit of a cold spell in front of goal coming into Matchday 18 away to Bournemouth, managing only a solitary strike across back-to-back draws against Everton and West Brom.
"The team I picked, in hindsight, was not the right team", Pardew said.
"Against Liverpool, who are so good on the counterattack, you can't turn the ball over like we did today".
Liverpool dominated the match and would have won by a greater margin had Coutinho not seen an 11th-minute free-kick hit the inside of the post and rebound across goal.
Salah became the first English Premier League player of the season to reach 20 goals in all competitions.
Despite that, Liverpool made a bright start and nearly took the lead after 11 minutes when Coutinho's curling free kick came back off the upright.
Dejan Lovren doubled the lead with a close-range diving header six minutes later and Salah made it 3-0 one minute before halftime by skipping past fullback Charlie Daniels in the right channel and beating goalkeeper Asmir Begovic with a neat left-footed shot.
Two down with only 26 minutes on the clock, Bournemouth were already on the ropes.
From the resulting corner, Lovren dived to head home after Firmino had somehow kept the ball in play.
Junior Stanislas' arrival to replace the injured Josh King finally sparked some life in Bournemouth, and the 28-year-old quickly sent Jermain Defoe haring through on goal.