Coutinho laid on the opening goal for Daniel Sturridge at the London Stadium before going on to score two of his own as Liverpool thrashed the Hammers 4-0 to move to within one win of a top-four finish.
Liverpool will look to seal Champions League qualification at home to Middlesbrough on Sunday, and Lallana believes that Coutinho could be key once again in unlocking the visitors' defence.
"Philippe is a massive player for us. In big moments in big games you turn to your main man and he was pivotal for us. His ball through to Daniel for the first goal unlocked the door and then two moments of brilliance with his finishing assured us the three points. He's been fantastic this season," he told the Liverpool Echo.
"Middlesbrough at home is a different challenge. In this type of game we've struggled to find the key to unlock teams at times.
"The reins are off them. Does that mean they are going to play with a bit of freedom or are they going to sit in? We need to adapt to however they play but more importantly we need to back ourselves to deliver."
Liverpool have finished in the top four of the Premier League table just once in the past seven years.