When Jadon Sancho began to feel the tiredness creeping in on Friday night it was hardly a surprise he clutched his hamstring and stretched his groin. He had hardly stopped running all night.
It was another desperate night in a season of disappointments, but Sancho was one of the few positives. His Manchester United career has been a slow-burn so far but over the next four months he's going to have a prominent role to play, so this was an encouraging evening.
It looked like even that bright spark would end up turning into a negative when Sancho signalled he had to come off with the final act of normal time, but he carried on for another 10 minutes before he could run no longer. Ralf Rangnick confirmed after the game that was it tiredness, rather than an injury, that had ended the 21-year-old's night.
United will need to get the cotton wool out before Tuesday's trip to Turf Moor and the Old Trafford fixture with Southampton next weekend. The options out wide looked like a strength at the start of the season, but now Mason Greenwood is unavailable and Anthony Martial and Amad have left on loan.
Sancho looks like a surefire starter almost every week between now and the end of the season. Marcus Rashford and Anthony Elanga can compete for the other spot out wide, with Jesse Lingard all in reserve. Rashford's form has fluctuated recently, Elanga is still raw at 19 and Lingard has been peripheral this season. United need Sancho to step forward now.
He struggled initially after his £73million move from Borussia Dortmund, finding it difficult to adapt to the goldfish bowl of life at Old Trafford and the freestyle nature of United's attack under Ole Gunnar Solskjaer. Sancho was a star in a well-drilled attacking system at Dortmund, but United's attack didn't have that structure.
It looked like his breakthrough would arrive with the departure of Solskjaer. Sancho's best performance to date came in the win at Villarreal that immediately followed it, a night when he broke his duck for goals or assists with a fine finish.
The appointment of Rangnick, a coach with similar principles to those that had helped the former Manchester City winger thrive in the Bundesliga, was another positive, but after two starts in the Premier League Sancho was then dropped. His start on Friday night was his first in a month, so it's excusable that he tired.
A lack of confidence has looked to be an issue for Sancho in a United shirt, so it might be no surprise that what might have been his most consistently threatening performance so far came after he'd started the game so well. He was unlucky not to give United the lead inside the first couple of minutes.
The ball landed at Sancho's feet after a blunder from Joe Lumley in the Boro goal, but the inventiveness and execution of the chip that followed was excellent. It felt like time was going in slow motion as the ball floated down straight onto the crossbar.
That set Sancho up for a fine performance, one where his appetite to run at players shone out, something he hasn't always embraced so far. His goal was created by a fine pass from Bruno Fernandes but Sancho controlled it expertly and showed the belief to take his man inside the area.
In the second half, he created excellent chances for Marcus Rashford and Cristiano Ronaldo while the score was still 1-0. Had either been taken it would have ended the contest.
United will now spend a couple of days hoping Sancho's weary muscles are ready for the challenge of a trip to Burnley on Tuesday. There is a narrative that comes with a game at Turf Moor, even if Burnley's home record doesn't really stand up to that scrutiny recently.
But it will be a different kind of game to those in which Sancho so often thrived in Germany. He's beginning to look more at home back in the Premier League and United need to see the best of him between now and May. The time for settling in is over.
