England won 1-0 on thanks to Bukayo Saka's first international goal, but a late injury for Alexander-Arnold may have spoiled the night.
Alexander-Arnold was left out of a previous England squad in March, but has made the 26-man cut for Euro 2020, with Gareth Southgate taking four right-backs.
Two of those right-backs – Kyle Walker and Reece James – were unavailable in Middlesbrough following their Champions League final exploits, so the Liverpool star got the nod.
However, in the closing stages, Alexander-Arnold pulled up after making a clearance with his left foot, and appeared to be clutching his thigh as he limped off the pitch, with England's medical staff then having to help him walk around the sidelines to the tunnel.
With England kicking-off their campaign against Croatia on June 13, Alexander-Arnold's prospects do not look good, but Southgate, who also has concerns over the fitness of Jordan Henderson and Harry Maguire, will wait for a full diagnosis before making a decision on whether to replace the 22-year-old.
"We're going to have a look. It's obviously not good to see him come off in the way he did. We'll just have to assess him over the next 24 hours," he told ITV Sport.
"I think thigh, but the medical team are still assessing it really. We just have to see, it's not a good sign to see him have to walk off as he did, but we'll know more in the next 24, 48 hours.
"Let's see how Trent is, and we'll go from there. We don't know the full extent, so the last thing you want is to have to see him come off in the way he did."
Another player to provide something of a scare to England fans might have been Jack Grealish.
The Aston Villa talisman missed a large chunk of the Premier League run-in due to an injury, but played over an hour before being replaced by debutant Ben White.
Grealish was seen with an ice pack on his right shin as he watched on from the stands, but the 25-year-old insisted he is fit.
"I've had an injury recently so to get 70 minutes under my belt, I'm glad," he explained to ITV Sport.
"It's not shin splints, actually, it's different to that, it's like a bit of stress on my shin, a different type of injury.
"I'm not concerned, I'm still aware of it, need to keep on top of it, do whatever I can to heal it after playing in these games, so that's why the ice was on."