Early in the derby against Everton, the accident happened to Virgil van Dijk.
Goalkeeper Jordan Pickford pulled down the midfielder in the penalty area.
The game was called off for offside, so the goalkeeper escaped warning.
However, Virgil van Dijk made a disastrous appearance.
The midfielder had to cancel the match after just over ten minutes.
Today, Liverpool confirm that van Dijk has suffered a serious knee injury.
The cruciate ligament is off and surgery awaits.
He will be away from football for a longer period.
The derby between Liverpool and Everton ended 2-2.