Oxford United chairman Kelvin Thomas has accused the players of not being fit enough for the promotion run-in at the end of the League Two season.
The U's missed out on a play-off place by four points after failing to win any of their last seven games.
And Thomas says fitness was a key factor in their slump in form with Crewe pipping them to the final spot.
"We do have some concerns, if I'm honest, in terms of our strength and conditioning," Thomas told BBC Oxford.
"Some of the factors do indicate that maybe they weren't as fit as they should have been, or as strong as they should have been, especially going into the last couple of months.
"It is an area that we are looking at quite closely."
Meanwhile, Dean Morgan, Scott Rendell and Cristian Montano, who were all on loan until the end of the season, have returned to their respective clubs.
Five players have been told that they will not be offered new deals with the club when their contracts expire this summer: Mark Wilson, Oli Johnson, Aaron Woodley, Paul McLaren and Anthony Tonkin.
Three players have been told that they will be offered new deals with the club: goalkeeper Wayne Brown and midfielders Asa Hall and Adam Chapman.