Vertonghen had smashed his nose into the back of team-mate Toby Alderweireld’s head when both players tried to clear a ball out of the box.
The big Belgian immediately went down with blood pouring out of a cut on the bridge of his nose.
He was taken off by Tottenham medical staff before being deemed okay to come back on the pitch.
The referee had a quick check of Vertonghen’s welfare and the former Ajax man duly made his way back on to the field of play.
Mere moments later, however, Vertonghen signalled that he needed to come off the pitch.
Mauricio Pochettino looked furious about something on the touchline and gestured towards the referee before putting his arm around his stumbling defender.
Vertonghen’s left leg appeared to go limp as two of Spurs’ club staff gathered round to help him stand up right.
Tottenham fans showed their appreciation for their stalwart centre-back as he was helped down the tunnel.
Gary Lineker, who is working the game for BT Sport, had his say on half-time, as did pundit Glenn Hoddle.
“Jan Vertonghen, a bit concerning how he reacted when he went back on the field of play and then came off,” Lineker said.
“He seemed fine there and then he kind of lost his legs and everything. Whether he kind of fainted.
“He’s okay in the dressing room apparently.”
Hoddle then had his say, noting that Vertonghen’s right leg had clearly gone limp as he was being helped down the tunnel.
“Look now, it’s horrible to see. He’s absolutely fainted, he’s gone, his right leg,” the former England manager said.
Tottenham spent the first half of the game chasing shadows, as Ajax showed them just why they’d made it so far in the competition – much to everybody’s initial surprise.
Midfielder Donny van de Beek put the four-time European Cup winners 1-0 up with a fine finish after being played a brilliant through ball.
