Ole Gunnar Solskjaer told his Manchester United players to stay fit in the off-season or risk not joining the team on their pre-season trip.
United complete what has been a miserable campaign at home to Cardiff City on Sunday, having already missed out on a top-four Premier League finish.
They will head to Australia, Singapore, China and Wales for pre-season friendlies beginning in mid-July and Solskjaer said any players returning out of shape could be left at home.
"The players have all got their programmes to do over the summer. We expect them to be ready to go on July 1 when we start," he told UK newspapers.
"We're not going to use the first 10 days to build them up. They should be ready to go. We've got a longer summer break than any of the other challengers and we need to use that as an advantage.
"It's not like a threat, but whoever is not ready on July 1 will probably stay here and not travel with us because we only want players we can use over there."
United head into the final day of the Premier League season 29 points behind leaders and rivals Manchester City and 28 adrift of second-placed Liverpool.
Solskjaer wants his senior players to set the tone next campaign and feels a strong start could see United begin to close the gap.
"It's down to me and the coaching staff. We drive this, we lead them on, but we want the players to be the main core of the surge forward, and we've got some experienced lads," he said.
"And we've spoken to them to say, 'We expect you to take charge next year'. We want them to, because we trust them.
"It's about getting momentum as well. The start of the season is going to be massive for us. If we can get the momentum early on, I'm sure we can catch a few teams.
"Our supporters know football. They know that teams go in cycles at times. Now it might be the start of a new team and era."