Chelsea legend Gianfranco Zola has admitted his sadness over the club's ongoing transfer dispute with striker Diego Costa.
At the end of last season Blues manager Antonio Conte informed Costa that he would not be part of his first-team plans going forward, clearing the way for him to find a new club over the summer.
The 28-year-old was unable to seal a move away from Stamford Bridge before the window closed, however, with his preferred move to Atletico Madrid an impossibility due to the La Liga side's transfer embargo.
Costa has since returned to his native Brazil and refused to report for training with Chelsea while the issue of his future remains up in the air.
"Nobody doubts the quality and value of the player but something must have gone wrong in the changing room," Zola, who played up front for Chelsea for seven years, told talkSPORT.
"A manager who is day in, day out in the changing room knows how important a player is, not only on the pitch but also within the group, so if Conte has come to this decision it means probably there were no guarantees any more to have that harmony in the changing room.
"It is big loss for the club because we know how important a player like Diego can be but you have to pay respect to the manager, who has to look after 24 players, not only one."
Costa is currently believed to be in the Spanish capital attempting to force through his transfer to Diego Simeone's side.