Premier League clubs and their managers have been left unsure on their players' true fitness, despite attempts to monitor their workouts remotely.
Coaching staff using tracking devices are concerned about receiving false readings, because treadmill running does not compare to that in a field.
They are having to take data 'with a pinch of salt' as a result, Sportsmail has been told.
This is likely to prompt clubs to call for a longer pre-season period before the Premier League action gets going again.
One sprint coach who works with footballers told Sportsmail: 'The added concern now is players have been told the season won't return until May 16 at the earliest.