But the good news is that the Uruguayan star is back on his feet after spending Saturday night in hospital and, according to boss Glenn Hoddle, he is already 'getting back to his normal self'.
Glenn even suggested that there might be a chance for the midfielder to return to action in next Monday's televised clash against Bolton.
Gus was concussed after clashing heads with Trevor Sinclair as he set up Les Ferdinand's winner at Upton Park.
"It's not as bad as we first feared," reported Glenn this afternoon. "The most important thing was to get him scanned and find exactly what was going on because he was totally out of the ball game, didn't have a clue where he was and there was concern.
"There is no brain damage, he spent the night in hospital and he's back home now.
"He's bright and breezy, I spoke to him yesterday and he's getting back to his normal self again.
"He'll have a bit of a bruise and a headache for a while but it looks as if he could have an outside chance of playing against Bolton on Monday.
"But he's out of Fulham. There is no way we could ask him to play there. It was too much of a blow to head. He has to be checked by the neuro-surgeon again before he gets the all clear."
Gus will join cup-tied Dean Richards on the missing list at Craven Cottage on Thursday night. Glenn added that Mauricio Taricco, ruled out of West Ham with an ankle injury, was 'improving all the time' and had 'half a chance' for Fulham.
Gus Poyet is out of Thursday's Worthington Cup tie at Fulham after suffering serious concussion in Saturday's win at West Ham.