Jürgen Klopp has said the increasing demands of international football have to stop or clubs will be “collecting the bones” of players throughout the season.

The Liverpool manager had to order Jordan Henderson and Trent Alexander-Arnold to take a post-World Cup holiday after the England internationals requested an early return to pre-season training. Henderson and Dejan Lovren, a World Cup finalist with Croatia, are likely to miss the Premier League opener against West Ham United on 12 August because of their late returns.

Another World Cup star, the Brazil goalkeeper Alisson, passed his medical on Thursday evening as his £66.9m move to Anfield edged closer.

Klopp has no issues with the World Cup – although believes it could have a serious impact on Tottenham Hotspur’s start to the new campaign – but the introduction of the Uefa Nations League in place of international friendlies is a concern for him. The new competition starts on 6 September, with the finals taking place next June, and he fears the constant demand on international players will have serious consequences.

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Klopp said: “I don’t want to get too football political but if they don’t stop with these games – they are now making it the Nations League to make it more important – then all these players who played at the World Cup have to go there again [in September]. I don’t understand that 100%. We had four players in the semis which is OK, we can deal with that. Tottenham had nine players in the semis. I have no clue how they will do it.

“It’s a really tough thing to deal with and in the future we have to change it. We have to change it. They play the World Cup, then the Nations League, next summer is the final of the Nations League which is really crazy. Everything is now so important in football and we have to collect their bones afterwards, more or less.”

Henderson and Alexander-Arnold both asked to resume pre-season training shortly after their World Cup commitments with England but the Liverpool manager rejected their requests.

“It was a hard fight to convince Jordan he needed a holiday,” Klopp said. “I knew that would happen. He feels like he needs training every day, a bit like Millie [James Milner]. On the phone I said: ‘That’s crazy.’ Jordan needs a holiday so that means 5 August he will be back. Dejan played a day later and had a party in Croatia so he will be back on 6 August. That would mean five days of training after three weeks of holiday. I hope we are in a situation where we don’t need to think about using these two against West Ham.

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“With Trent I have a special agreement. He wanted to be in earlier as well. As he’s very young I said: ‘OK, have two weeks’ holiday and then we’ll speak and see.’ If you asked Trent he would be good to join us in Evian [where Liverpool have a training camp at the end of July] but I’m not sure I will do that.

“I want to have them desperately but the season is really long. We have three games before the first international break. Then it will be the Nations League. It will be difficult to involve them against West Ham but I don’t want to make that lineup today.”