Mauricio Pochettino admits he is worried about Tottenham's future trophy credentials due to the mentality they showed in Saturday's 2-1 loss to Southampton.

Harry Kane put the visitors ahead in a first half at St Mary's Stadium that the visitors controlled, with Maya Yoshida also turning the ball against his own post and Christian Eriksen rattling the crossbar with a free-kick.

However, Spurs failed to live with an improved Southampton after the break and goals from Yan Valery and James Ward-Prowse condemned them to a third defeat in four Premier League games.

Pochettino, who served the first of a two-game touchline ban, struggled to understand his side's second-half display at the end of a week in which they secured a Champions League quarter-final spot at the expense of Borussia Dortmund.

"Reaction? Mine is like yours," he told a news conference. "Difficult to understand. The way we started the second half, the way we gave the belief to Southampton... So disappointed. I don't understand why we changed the way that we changed. Too difficult to accept. It's a thing that worries me.

"I am so critical with myself and critical with my team. We want to fight for these things. Quarter-final of a Champions League – fantastic. But if you play like that in the first half, then play like that in the second half, we're not in the best eight of Europe. I am so, so, so disappointed. We are now going to have three weeks. It's difficult to take the feeling we've got today."

The result means Manchester United will join Spurs on 61 points if they beat Arsenal on Sunday, while Chelsea can also move level if they win their two games in hand.

"It's good, a lot of people now are going to realise we are level and maybe start to talk about perception," said Pochettino. "It's important now to know that we need to fight a lot until the end of the season.

"I'm going to tell what I told before: I am a little bit worried [about] this change. From the first half to the second half, it changed too much. It's only one reason - it's mental. I don't want to blame someone, I want to blame all of us, the club.

"We are in a very good position but we're disappointed in the last few games, but we are still in a position since the beginning of the season. Maybe we need to understand it's our limit to be there, but of course if that's our limit we need to make some decisions in the future.

"If we want to keep our position in the top four, we want to fight in the quarter-final, we need to give more in the future if not we are going to struggle. We can't only compete for 45 minutes and then after 45 minutes say, 'okay, we are going to lose'. Southampton will punish you."

Pochettino feels the result shows Spurs must accept they are not yet among the top sides in the world, with the club announcing this week that their new stadium will finally be ready to host matches next month.

"That shows that we still have a lot of work to do. That shows we're not mature enough. That showed that to be in the last level like a club, we need more effort, we need more quality, [to] increase your capacity of commitment," he said.

"Everything you need to increase on the last level to be the best team, the best club in the world. That's why we're talking a lot in the last four and a half, five years. It's not easy to build a plan, strategy, to arrive and to be one of the best clubs in Europe or England.

"It's so easy to talk when we have good performances that we are the best team. I think we need to be careful at how we assess ourselves. In the last few years, we are living more in perception of reality, now to say is reality. To be consistent, you need time, if you want to be a real contender in the future."