He returned to the provisional 33-man squad last week and made the cut for the Euros as one of four right-backs alongside Kieran Trippier, Kyle Walker and Reece James.
Now he will watch from home as Southgate backed him to return from the setback and revealed the injury should not prevent Alexander-Arnold from starting the new campaign with Liverpool.
“It’s heartbreaking, really, for any player to get to the eve of a major tournament, be named in the squad and then miss out through injury,” Southgate said.
“You know how rare these opportunities are and even though he’s a young player who is going to have those opportunities again – I firmly believe that – that’s a really difficult moment for him especially.
Talking points ahead of England s final Euro 2020 warm-up
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Timelines getting more difficult - Gareth Southgate explains fitness blow for Liverpool captain Jordan Henderson
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What s been said?
The England camp have released an official statement confirming the news, which reads: Trent Alexander-Arnold has been ruled out of UEFA EURO 2020.
The Liverpool player sustained a thigh injury in the closing stages of Wednesday’s 1-0 win against Austria in Middlesbrough.
A scan on Thursday confirmed the 22-year-old will be unable to play in the forthcoming tournament so he s withdrawn from the England squad.
Gareth Southgate will not confirm a replacement until after Sunday’s return to the Riverside Stadium for the warm-up fixture against Romania.
Liverpool have released their own statement revealing that the defender will now return to Anfield to begin his recovery.