Mauricio Pochettino defended his medical staff at Tottenham after the club’s 1-0 Champions League semi-final, first-leg defeat to Ajax was overshadowed by a sickening head injury to Jan Vertonghen which raised questions about concussion protocols.

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The Spurs centre-half was laid out in the 32nd minute following a free-kick into the Ajax box which saw him challenge with the Dutch team’s goalkeeper, André Onana, and his Spurs colleague Toby Alderweireld.

Vertonghen’s nose was bloodied and he needed lengthy treatment, together with a full kit change, before he was given the all-clear by Spurs medics. The Spanish referee, Antonio Mateu Lahoz, demanded that Vertonghen be double-checked, which he was, and he was allowed to return but moments later he became unsteady on his feet and signalled that he had to come off.

In distressing scenes, Vertonghen retched and half-collapsed before he was helped down the tunnel. By the time that Pochettino introduced the substitute, Moussa Sissoko, it was the 39th minute.

Facebook Twitter Pinterest Jan Vertonghen was withdrawn for Tottenham after clashing heads with teammate Toby Alderweireld. Photograph: Tom Jenkins/The Guardian

Spurs said that the Belgian had passed all the on-field concussion tests, as per protocol – in other words, he had not suffered a concussion. He was later able to walk away from the stadium, briefly stopping to tell reporters he had not broken anything and there had been no concussion. Vertonghen said he had felt weak and briefly fainted.

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Pochettino said: “I was not involved. It was the doctor’s decision. It’s so important that the rules and the protocols are there and our medical staff followed the protocols. The referee asked and the action we needed to take was to take him out because he did not feel well. Now he is OK, now he is good. He was walking away [from the stadium], he was more relaxed. You need to keep eyes watching him and monitor him because it was a big knock but at the moment he is OK.

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“First of all, we must protect the player’s integrity [health]. Our medical staff followed the protocol and they decided it was possible to restart the game and possible for Jan to play again. But, of course, Jan started to feel unwell and we needed to change him.

“Of course I was worried. That is normal because for me the most important is the health of the player. If they say to me ‘change’, I am not going to doubt that. In that type of situation, the medical staff are the bosses. Never am I going to question them.”

Facebook Twitter Pinterest Jan Vertonghen was assessed by Tottenham’s medical staff and had to change his bloodied shirt before taking the field again. Photograph: Tom Jenkins/The Guardian

Spurs face a difficult task to turn the tie around in next Wednesday’s second-leg after they started slowly and fell behind to Donny van de Beek’s goal. Until Sissoko came on and Pochettino changed from his back-three system, his team were second best. They improved thereafter but they struggled to create many clear cut chances.

“The team didn’t show the energy that we wanted to show,” the manager said. “Ajax showed more energy than us and the way we conceded the goal was so painful. We were so poor in this action.

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“In the first half hour of the game we struggled because they showed more energy, they were more proactive than us. That was the key. With Sissoko on, we started to play better, with more energy. The second half was much, much better.

“We pushed them to play deeper and we started to press in the way that we wanted to press. We started to play more and I think the second half gives us hope for the second-leg in Amsterdam. The tie is still open, although it’s going to be difficult.”

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The Spurs midfielder Christian Eriksen, a former Ajax player, added: “We were under-par and we didn’t play our best at all. In the first 20 minutes we were ball-watchers. We need to change it up a lot for the next game. We made them look a lot better. They are a good side but we helped them along the way. It was our fault.

“At the start the system didn’t matter. The change gave us a different way to press, to play more direct. But we were still far from where we know we can be. No one wanted to play the first half we did. Everyone knew we didn’t compete. We were better in the second half.

“We can’t keep talking about injured players. In a semi-final it doesn’t matter who plays. We have to step up. We’re lucky they hit the post [in the second half] and, hopefully, we can turns things around in Amsterdam.”