Mason Greenwood's omission from Manchester United's squad against Everton on Saturday followed a poor display in training the previous day, it has emerged.
Ole Gunnar Solskjaer, the United boss, blamed Greenwood's absence on illness but it is understood the 19-year-old's sluggishness and application in Friday's training raised concerns.
Bruno Fernandes, who scored twice in United's 3-1 win at Goodison Park, was thought to have been left exasperated in one instance when Greenwood failed to run on to a pass from the Portuguese but United denied any knowledge of such an incident.
"He wasn't feeling well last night," Solskjaer said after the game. "He looked well in training, he looked OK, but he reported not well and couldn't travel."
Greenwood was left out of United's squad against Newcastle and Paris Saint-Germain last month. Solskjaer initially put Greenwood's absence down to a "niggle" but later cited illness for the omission.
Concerns about Greenwood have been growing for some time. One issue is understood to centre around his recovery between matches amid worries he is not getting enough sleep, particularly at a time when the congested fixture list has made it increasingly important for players to rest and look after their bodies as best as possible.
Solskjaer has already complained about the fixture scheduling and refusal to permit five substitutes, increasing the threat of injury and United are eager to do everything possible to preserve their players' fitness. In a separate matter, Solskjaer has denied Greenwood has had an issue with his timekeeping.
Troubling reports about Greenwood's private life emerged in the aftermath of the teenager being kicked out of the England camp by Gareth Southgate in September after he and Manchester City midfielder, Phil Foden, invited two women to the team hotel in Reykjavik the day after a 1-0 win over Iceland.
Greenwood - who is widely regarded as one of Europe's most exciting young attackers - and Foden were subsequently axed from the England squad for the triple header against Wales, Belgium and Denmark last month.
Foden was recalled to the England squad this month for the games against Ireland, Belgium and Iceland but Solskjaer and Southgate agreed during a phone conversation prior to the squad announcement that it would be in the best interests of Greenwood to sit out the fixtures.
United are conscious of Greenwood's age and inexperience and the demands and attention that come with playing for the club and hope he can draw inspiration from some of the other talents, such as Cristiano Ronaldo and Ryan Giggs, who also had to contend with an intense spotlight from a young age.
Meanwhile, France coach Didier Deschamps has claimed Paul Pogba is unhappy about his position in United's team.
Deschamps said: "He is in a situation with his club where he cannot be happy, neither with his playing time, nor with his positioning. He needs to find his rhythm."