The debate over whether he would be picked by Gareth Southgate has rumbled on for months but it was only a recent injury to Chelsea's Reece James which made the 24-year-old's selection more likely.
Following confirmation of his place on Thursday, Klopp said Alexander-Arnold would be an asset in Qatar.
"I am not sure he was in doubt but with all the discussions - at least the discussions I followed - if everyone had been available it was likely you don't take four right-backs," said Klopp.
"I don't know exactly why we have this discussion but it's fine. When you have the choice between four or five - and (Kyle) Walker is kind of fit and Reece James obviously is not available - I think Gareth is very happy he has Trent as well.
"Trent took it, he is very calm. He plays the football he plays, tries to improve, tries to develop, but I think he would have been disappointed if he had not been there."
Klopp has steadfastly, and unsurprisingly, backed his player to perform at the highest level despite continued criticism about his defensive vulnerabilities against the very best opponents.
The Liverpool boss has always stressed it is more important to focus on what Alexander-Arnold can do rather than what he cannot.
However, he believes the player's past performances stand up to scrutiny about his capabilities in big-stake games, having become the youngest player to start three Champions League finals (all in five years) when he was still 23 years and 233 days old.
"There is a lot of talk around these kind of things," added Klopp.
"I heard now Gary Neville said something about him not being able to play in the knockout games but Trent Alexander-Arnold is now 24 and he won quite a few finals, that is a very important game where you have to defend.
"We played (Carabao and FA Cup) finals against Chelsea last year, both finals were incredibly intense, top-class players, world-class players in moments better than players you face in the World Cup, like in the [Champions League] finals we played against Tottenham and Real Madrid and he always was there, always defended well."
The final Premier League match before the World Cup sees Southampton visit Anfield for new manager Nathan Jones' first match in charge.
Alexander-Arnold is one of just seven Liverpool players who are heading to Qatar but with the tournament so close Klopp does not expect any of them to hold back against Saints as they try to close the gap on the top four.
"We are professionals and the boys played the game like they always play the game, with full focus. Everything can happen in the football game," he said.
"But if you go to the World Cup and think you have made it and get injured in the first game I am not sure which is the biggest disappointment.
"To perform at the highest level, you have to focus on what you need to do in the moment and you cannot think about the next possible thing.
"We know this is incredibly important, this is our World Cup, the Southampton game.
"We had a tricky first part of the season but if we can win this game we could make 22 (points) and it could give us a distance where we can see light at the end of the tunnel and that is exactly the mindset all the boys have as well.
"We don't make a big fuss of it like I have to ask everyone every day 'How are you?', we don't talk about the World Cup.
"The boys who were nominated, great, some others were not and that is not cool but apart from that it is just our situation here and that means Southampton."