Thomas Tuchel said Phil Foden's place in England's World Cup squad is "not a guarantee" after failing once more to hit peak form for the Three Lions.
Foden struggled in an unfamiliar role up front in the absence of Harry Kane in England's 1-0 friendly defeat to Japan on Tuesday.
After a bright start to the season, the 25-year-old has also slipped down the pecking order for Pep Guardiola at Manchester City in recent weeks.
"He tried everything. I would say he was excellent in camp but, yeah, he struggles to show it on the pitch," said Tuchel.
"Obviously he didn't have a lot of minutes for City recently, then he came to camp with the brightest smile and was so good in training.
"And I thought he will just surprise us and will play with the same verve and excitement but he struggles to have the full impact."
Foden has rarely taken his best City form on to the international stage, even on the run to the Euro 2024 final fresh from collecting Premier League player of the year awards.
And he faces stiff competition from Jude Bellingham, Cole Palmer and Morgan Rogers for an attacking midfield berth in Tuchel's squad.
"It's not a guarantee that he will come," added Tuchel when asked if he can take a struggling player to the finals in North America.
Defeat to Japan was just Tuchel's second in 12 games as England boss, but he is yet to beat a side ranked among FIFA's top 20 over a year into his reign.
After also drawing 1-1 against Uruguay last week, the German said a difficult March "will not define us" as he remained confident England can end their wait since 1966 to win a major tournament this summer.
"We will not let go of our dream, we will not let go of the question 'why not?'" said the former Chelsea boss.
"Now the most important thing is that the players reintegrate with their clubs, have a good end of the season, and then we have them in pre-camp and prepare them properly and take steps from there."