Jan Vertonghen sustained a nasty head injury during Tottenham's Champions League encounter with Ajax.
The Belgian defender was left bloodied and required lengthy treatment after colliding with team-mate Toby Alderweireld and Ajax keeper Andre Onana as they competed for a Kieran Trippier free-kick.
Vertonghen tried to return to the field of play but quickly indicated he needed to be replaced with Moussa Sissoko coming on in his place.
He was helped off and headed straight down the tunnel to be assessed by medical staff.
Boss Mauricio Pochettino said: "I don't know [how he is]. We are going to assess him in the next few days."
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Tottenham will have it all to do in Amsterdam next week if they are to keep their Champions League dream alive as Ajax claimed a 1-0 win in the semi-final first leg.
Spurs went down to Donny Van De Beek's first-half goal at the Tottenham Hotspur Stadium.
This was just their second ever European Cup semi-final, and 57 years since their last one, but they are not out of it as they showed that this excellent Ajax side can be got at, even if they did not force goalkeeper Andre Onana into a meaningful save.
They will have Son Heung-min, and maybe Harry Kane, back for the second leg and they will have no option but to go for it in the Dutch capital.
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Rio Ferdinand on BT Sport: "It was professional and mature from Ajax. In the first half they played fantastic football.
"In Spurs defence, if you take 44 goals out of the side in Harry Kane and Son Heung-min, any team would suffer.
"I do think they are still in it. Their away form in this tournament shows they are capable of getting a result."
