The West Ham midfielder produced an inswinger and Liverpool goalkeeper attempted to punch the ball away under a challenge from Angelo Ogbonna.
However, the Brazilian couldn’t get anywhere near enough on the ball and the ball ended up in the back of the net.
There was a VAR check to see if the West Ham defender had impeded Alisson but VAR decided he hadn't.
It was an incredible start to the match as West Ham looked to leapfrog their opponents in the table.
VIDEO: Alisson's own goal vs West Ham
Liverpool are yet to lose this season and were looking to go within one point of Chelsea at the top of the table, who dropped points at home to Burnley on Saturday.
But they got off to the worst possible start after Alisson failed to deal with Fornals' corner.
Ahead of the match, Liverpool manager Jurgen Klopp admitted that the busy schedule his side are currently in the middle of means they're not able to train as they'd like.
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"It's not possible to keep the players fresh, the schedule is against us, you need luck, we actually don't really train anymore, we just recover," the German coach said after Liverpool beat Atletico Madrid 2-0 on Wednesday evening in the Champions League.
"The boys who played tonight, they recover until we play West Ham, the boys who didn't play, they train a little bit, that's clear, but when you play in this rhythm it's pretty much impossible to keep team fresh, you just fight through."News Now - Sport News