Ole Gunnar Solskjaer has ordered his Manchester United players to report for preseason training as early as July 1 after being dismayed by fitness levels following his arrival as Jose Mourinho's successor, sources have told ESPN FC.
With the bulk of United's squad facing an extended summer break due to only a small number being likely to be involved in the Copa America, Solskjaer has set the early return date in order to ensure they are fit enough to compete for the top four next season.
Eric Bailly, meanwhile, will miss Ivory Coast's African Nations Cup campaign with a knee injury.
Sources told ESPN FC that Solskjaer and assistant Mike Phelan were struck by the unsatisfactory fitness of the majority of the players when they were given interim control of the team in December.
The squad were then subjected to a tougher fitness regime that coincided with the upturn in results that saw United win 10 of Solskjaer's first 11 games in charge.
However, concerns that they would suffer a reaction to the increased demands proved well-founded, with United's results nose-diving during the final weeks as the club failed to qualify for the Champions League.
"The players weren't fit enough when Ole arrived," a source told ESPN FC. "They worked really hard to change that in December and January but they hit the wall in the end, and that was probably inevitable."
United failed to win any of their final five league games as they finished five points adrift of fourth-placed Tottenham.
While Solskjaer is determined to overhaul his squad, with at least three new signings and emerging youngsters expected to replace those heading for the exit, he also believes his players must vastly improve their fitness.
United are due to play their first preseason game against Perth Glory in Australia on July 13, but the players will be exposed to a tougher regime when they return at the beginning of that month.
Meanwhile, the club will discover on Saturday whether they must begin their Europa League campaign on July 25 -- and face the prospect of six qualifying games before reaching the group stage -- after finishing sixth in the Premier League.
If Watford defeat Manchester City in the FA Cup final on Saturday, they will have to enter the competition at the second qualifying round stage.
That would play havoc with their preseason plans, with arrangements already in place to play Tottenham in Shanghai in the International Champions Cup on July 25.