Mauro Zarate's Watford career has suffered a setback after he sustained a cruciate ligament injury against West Ham on Saturday.
The Argentine returned to the Premier League in January when he joined the Hornets from Fiorentina, and went up against his former employers at the weekend.
But Zarate, who won the penalty that led to his team's goal in the third minute, had to be replaced just before half-time after twisting his knee.
Subsequent scans revealed the worst, with the 29-year-old confirming the extent of the damage, which could keep him on the sidelines for six months at least.
Zarate took to Instagram to announce he had suffered an "anterior cruciate ligament rupture plus an injury grade one of [the] medial collateral ligament."
He added: "Thank you everyone for all the messages in this horrible moment. I will face this with all my strength and will be back stronger than before."