Liverpool fans angry with Jurgen Klopp tactical change after Lionel Messi goal

Liverpool's German coach Jurgen Klopp puts on a headset during a press conference at the Camp Nou Stadium in Barcelona on April 30, 2019 on the eve of the UEFA Champions League semi-final first leg football match between Barcelona and Liverpool. [AFP]

Liverpool fans are angry with Jurgen Klopp's decision to bring Roberto Firmino on after Lionel Messi scored - with many saying the Brazilian should have already been on the pitch.

Klopp opted to leave the forward on the bench until the 78th minute against Barcelona, in the semi-final of the Champions League on Wednesday.

Ahead of the game the Liverpool boss confirmed the Brazilian was fit to play.

Klopp told reporters: "Bobby wants to [play] and the medical department told me 'yes, it's good' but then you have to make sure – I'm not sure if 90 minutes or whatever.

"So he doesn't have to start, that's why we start like we start. Bobby is on the bench, that's good news."

Klopp played Georginio Wijnaldum in Firmino's false nine role, but things just weren't working for Liverpool in the final third.

But Klopp delayed the decision, allowing Barcelona time to add a second.

On social media, Liverpool fans hit out at Klopp with one tweeting: "Why the hell did he not bring on Firmino? We needed someone who can produce magic."

Another added: "Maybe Messi wouldn't have had that chance had Klopp decided to put Firmino on when we were putting Barca under pressure and pinning them back."

And a third said: "Don't get Klopp tonight, I really don't. You have three subs to impact a game. They are playing on a huge pitch.

"We've dominated the match, it's just not quite clicking up top with Gini and he makes no subs. 2-0 down Firmino finally comes on 3-0 down Origi."