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Philippe Coutinho had to leave Liverpool's game at Watford early in the first half on Monday after picking up an injury.
The Brazilian star collided with Adrian Mariappa in the fourth minute, with the Watford defender's knee making contact with Coutinho's right thigh.
Coutinho limped off the pitch, then briefly tried to return before the injury proved to be too much.
Adam Lallana, who had missed the last five games with an injury of his own, came on in his place.