Jurgen Klopp "didn't understand" Gareth Southgate's decision to leave Trent Alexander-Arnold out of his England squad, claiming the Liverpool right-back's form "cannot be the reason".
Alexander-Arnold was a surprise omission from Southgate's group for last month's World Cup qualifiers against San Marino, Albania and Poland.
The 22-year-old has been a Liverpool regular in recent seasons but has earned only 12 England caps to date.
Kieran Trippier was preferred at wing-back at the 2018 World Cup, with Kyle Walker playing inside in a three-man defence.
The Manchester City defender and Reece James shared starting duties in March, while Trippier was also in the squad, with right-back a clear position of strength for the Three Lions.
As well as Alexander-Arnold and Southgate's current preferred trio, England could call upon Aaron Wan-Bissaka at Manchester United or England Under-21 pair Tariq Lamptey and Max Aarons.
But Klopp believes his player deserves to be at the forefront of the conversation.
"I was surprised when he didn't get a call-up," the Reds manager said.
"I'm a manager myself, I make decisions every day, every weekend, every midweek about team selections, squad selections. You always make decisions other people don't understand.
"I respect that Gareth makes his own decisions, of course, obviously, but I didn't understand it."
Alexander-Arnold is widely accepted to be enduring a more difficult campaign after playing a starring role in Liverpool's Champions League and Premier League successes in the previous two seasons.
The defender provided 15 assists in each of the 2018-19 and 2019-20 campaigns but has supplied just five this term.
He has created a relatively meagre 59 chances, down from 109 last season, his per-90-minutes average dropping from 2.46 to 1.87. His crossing accuracy has steadily declined over the past three years, too, to stand at 17.9 per cent in 2020-21.
But Alexander-Arnold still leads James (39), Trippier (29), Wan-Bissaka (26) and Walker (14) in key passes, while only the Atletico Madrid man has more assists (six).
On the other hand, Walker's defensive statistics are more impressive, having been dribbled past on just 12 occasions – Alexander-Arnold is at 22 – while not committing a single error leading to a shot.
Klopp continued: "Trent Alexander-Arnold was in the last two years at least the stand-out right back in world football at an unbelievably young age.
"I have no problems when my players get criticism – from me, from other people if they are not performing – but that's the truth.
"Now he started the season with COVID-19 early on, which is not good. As you can imagine, it took him a while to get in the season.
"From the moment he was in the season, he was one of our most consistent players, a standout player – Robbo [Andy Robertson] the same, who played at this high level.
"That cannot be the reason that he was not in the squad.
"If you compare Lionel Messi every year with his best season ever, you'd say he's not playing like he performed in 2005 and 2006, but he's still the world's best player."
Alexander-Arnold was left out due to England's style of play, Klopp theorised, and his club boss still felt he should have been included.
"If you have to play against a deep defending side, for sure, the crosses and the passes of Trent, I don't know anyone who can do them like him," Klopp said.
However, Alexander-Arnold's crossing accuracy ranks third among Southgate's five senior options and Walker, Trippier and James combined to find team-mates with a solid four of their 11 deliveries for England in March.
But frustrations with Southgate's selection at least created the opportunity for Klopp to work with Alexander-Arnold in training ahead of the run-in.
"Can he get the benefit? Of course," Klopp said. "He was not with the national team, so he had time to train, trained at an extremely high level.
"You could see he showed a reaction. There was no need, but he was really in the session; that was good. He really enjoyed the session.
"He's a very ambitious boy, so I'm sure he wants to be in the England squad. I know he wants to be, and he will try everything to get there again.
"If Trent Alexander-Arnold is not in the squad of England, England must have an extremely good squad. I'll say it like this. That's the situation.
"I respect the decision, of course I did, but I didn't understand it."
Standout matches today