That’s the view of pundit and former Reds midfielder Danny Murphy, who saw Liverpool beaten 2-0 in Serbia.
It was their second defeat in the Champions League and came with Firmino rested from the starting XI.
The Brazilian has struggled this term in comparison to last season, scoring just two in the Premier League so far.
Liverpool are unbeaten in the league and face Fulham today (12pm) as they look to heap the pressure on Chelsea and Manchester City.
Firmino is likely to return to the side for the clash against strugglers Fulham at Anfield.
And Murphy believes the loss to Red Star Belgrade showed Jurgen Klopp the worth of the forward.
“One thing we learned, if we didn’t know it already, is how important Roberto Firmino is to this Liverpool team,” Murphy wrote in his column for the Mail.
“He was rested on the bench and by the time he came on at half-time, Liverpool were already 2-0 down.
“He has done brilliantly at the club and impressed me immensely.
“Firmino is technically very good but what he relies on to be at the very top of his game is being physically right because his output is enormous.
“He is the trigger for all the pressing and his workrate is phenomenal.
“If for some reason he is having a dip, maybe because of tiredness, maybe a couple of aches and pains, you notice it.
“Some players can get away with it because they move the ball, Firmino is hard yards all over the place when he’s at it.
“To me, he hasn’t looked quite as sharp this season, and maybe that explains why Klopp didn’t start him in midweek, to give him that little break.
“Funnily enough, Romelu Lukaku is a similar type I that when he drops physically, it affects his performances. When he’s at it, he’s unstoppable.
“The good thing is that restoring top physical condition is the easiest thing to work on.
“It’ll only be temporary for Firmino and we saw in Belgrade, Liverpool are always a better team with him in it.”