The Uruguayan limped off after 35 minutes and the boss says the injury will be assessed today or tomorrow.
"It's too early to say really," said Glenn. "It's an ankle problem, not the one he was suffering with before but, I can assure you, for Gus to come off it must have been very painful.
"We won't know how much damage has been done until Tuesday or perhaps later."
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Gus Poyet suffered a painful ankle injury in the first half against Fulham, reported Glenn after the game.