At half time of Thursday's Europa League semi-final first leg against Villarreal, manager Jürgen Klopp elected to sub off Philippe Coutinho in favor of Jordon Ibe.
In a tactical sense, the move was curious. While Coutinho struggled to involve himself in the first half, failing to put a shot on target, he was still expected to play a crucial role in breaking open Villarreal's defense. Ibe, meanwhile, was ill-suited for the appearance and floundered in the second half.
It was reported by BT Sport after the match that Coutinho was taken off due to an undisclosed illness, clearing up the reasoning for the substitution.
The news comes as a relief at this stage given the Brazilian's battles with a hamstring injury over the past year, and Liverpool fans will be hoping that his reported sickness is nothing serious.
According to the Express, Coutinho is a doubt for this weekend's match with Swansea City. But until an official club statement comes out, it will be difficult to judge the severity of the situation.