The Belgian lines up in attack at Estadio do Dragao for the second leg of the Champions League quarter-final, which the Reds lead 2-0.
And Klopp revealed before kick-off in Portugal that Origi’s form and freshness, rather than any fitness issue with Firmino, prompted the switch.
“No issue, actually – just respecting the fantastic form Divock Origi is in for weeks or months already, and bringing in fresh legs,” the boss told BT Sport.
“That’s all, the same we do in midfield because we expect a very intense game, we expect a hard-fighting Porto side. So we need to be ready for that.