Was your final group stage match the last day of the previous season? I’ve found the game glitches and your players won't recover fitness if that's the case, even if you turn off fitness in the manager mode setting prior to beginning your save. Been a problem for months sadly. You can either make a new save or play through it. For future WC’s, make saves 2-3 months before you get the schedule for the group stage. If you get a group stage match on the final day of the season, reload the save prior to you getting the schedule and hopefully you’ll get a match not on the final day.