Manchester United's Luke Shaw has admitted that manager Louis van Gaal was right to question his fitness.
The England left-back, 19, says he underestimated how difficult the step-up from Southampton would be after his £27m summer move to Old Trafford.
Shaw said he "totally agreed" with comments about his fitness made by Van Gaal in a news conference this week.
"To play in this formation you have to be really fit. I am fit but I am not at the high level he demands," he said.
"I have got to get fitter to get up and down. It's something I have got to work on," added Shaw, who has been put on a special fitness programme by the United manager.
|World's most expensive defenders|
|David Luiz||Chelsea||Paris St-Germain||40||2014|
|Thiago Silva||AC Milan||Paris St-Germain||36||2012|
|Rio Ferdinand||Leeds United||Manchester United||33.1||2002|
|Luke Shaw||Southampton||Manchester United||27||2014|
|Fabio Coentrao||Benfica||Real Madrid||27||2011|
He only played in Tuesday's penalty shoot-out win over Inter Milan in the International Champions Cup in Washington because youth product Reece James pulled out of the match with injury.
Shaw says the demands of preparation and training at United were "a surprise".
"I didn't come into United thinking it would be much harder," he said. "That's something I think I made a mistake on."
Shaw, the world's fourth most expensive defender, was as a second-half substitute as United drew 0-0 with Inter before winning a penalty shootout 5-3.
"I felt great. I could have played another 45 so that's something that shows there is a progression there after the last game," Shaw added.
"The focus is now to get fitter and fitter and be flying by the start of the season."
Van Gaal reckons his side were good value for their win.
"We deserved the victory, I think," he said. "I am pleased not only because of the way we kept possession, but because of the quality of the opponent."
United now fly to Detroit for Saturday's match against Real Madrid, expected to take place in front of a 100,000 crowd. Real forward Cristiano Ronaldo could be fit to face his former club, with coach Carlo Ancelotti saying the World Player of the Year was recovering well from a knee injury.
The Champions League winners were beaten 1-0 by Roma in Dallas on Tuesday, Francesco Totti scoring the Italian side's winner.
A win on Saturday for United, who beat Roma in their first match, will see them qualify for the final of the friendly tournament on Monday, 4 August in Miami, where they could face Manchester City or Liverpool.