Valencia sporting director Jesus Garcia Pitarch has said he will not tolerate bad behaviour from midfielder Dani Parejo, who was banished from training for apparently trying to force through a move to Sevilla.
Sevilla have reportedly offered around €10 million for Parejo, but Valencia do not want to sell him to a direct rival.
Coach Pako Ayestaran was angered when Parejo did not want to warm up before coming on as a substitute in a friendly against Tampere.
The player was also reported to have been told to leave a team meeting because of a bad attitude.
Speaking at a news conference at which ex-Barcelona right-back Martin Montoya was presented as a new Valencia player, Garcia Pitarch said: "We will not tolerate this behaviour from any of our players, especially Dani Parejo as it means a big cost for the club.
"His behaviour has not been professional. He is disrespecting the Valencia fans and should apologise.
"I have never seen a player behave like this. I hope this ends soon, as we are not going to sell the player."
Parejo's behaviour was also criticised by former Valencia and Spain goalkeeper Santiago Canizares.
1.- La actitud del jugador es lamentable, máxime siendo uno d los mejores pagados de la plantilla y con la confianza de los técnicos
- SANTIAGO CAÑIZARES (@santicanizares) August 1, 2016
"The player's attitude is lamentable, especially as he is one of the best paid players in the squad and has the confidence of the coaches," Canizares wrote on Twitter.
"We will have to see what decision the club make. To leave him in the stands would be to pay his wages, not get the transfer fee and lose the player. It is not an easy situation. Maybe there are other offers from other clubs."
Valencia have already seen senior squad members Andre Gomes, Alvaro Negredo, Sofiane Feghouli, Rodrigo De Paul, Antonio Barragan and Pablo Piatti leave in the summer.
They are also reported to be willing to listen to offers for Arsenal target Shkodran Mustafi and Brazil goalkeeper Diego Alves.
Local press reports said they had turned down a €19m offer from the Gunners for Germany defender Mustafi.
Meanwhile, Valencia had made an enquiry about bringing their former centre-back Raul Albiol back to the club from Napoli, but were told the Serie A club do not want to sell.
"I won't do to others what I don't like when it's done to me," Garcia Pitarch said. "I spoke with [Napoli owner Aurelio] De Laurentiis in Naples and that option was closed off.
"The player is important for the team there and has a contract -- but nobody can complain about the behaviour of Raul Albiol at Napoli."