It came, seemingly, in an innocuous situation midway through the second half of Wednesday night’s victory over the Spanish champions.
Speaking to reporters after the game, Klopp revealed that it was a hamstring injury for Firmino, which almost certainly rules him out of Saturday’s clash with West Ham at the very least.
Milner is not expected back until after the international break with a similar problem, while no timeframe has been placed on Keita’s recovery after the issue he picked up in the 2-2 draw with Brighton at the weekend.
Losing Firmino is a big blow to Liverpool, particularly given his excellent performances of late, including a hat-trick in the 5-0 thrashing of Watford.
It is not the first hamstring injury the 30-year-old has suffered already this season either, having missed four games with a problem sustained against Chelsea in August.
Firmino was named in the Brazil squad to play Colombia and Argentina later this month, but he could now pull out of duty – having also missed the previous two international breaks.