It didn't look especially serious when it happened. Ashley Williams kicked out late, appearing to catch him in the torso. He tumbled, landing awkwardly but not with exceptional impact. And though he stayed down, for a time it seemed most likely he had simply been winded; that he would take a minute to compose himself before returning to the fray.
Then it became clear Philippe Coutinho wasn't winded; that he was favouring his shoulder. Then, seconds after returning to the pitch, he went down again, clutching it, and had to be helped to the sidelines to be substituted. Still, even though he went straight down the tunnel, it was difficult to imagine it was an especially serious injury.
Unfortunately for Liverpool and Coutinho, though, it turns out to have been far more serious than it seemed to have any right to be when it happened, and not only will the Brazilian attacking midfielder miss six weeks recovering from it—he'll have to undergo surgery to stabilise the acromioclavicular joint.
Coutinho will be out until the end of October at the earliest. As bad as that is for Liverpool, though, the return of Luis Suarez from suspension against Manchester United in the League Cup means that manager Brendan Rodgers would seem to have a ready made replacement who can step in following Saturday's match against Southampton.