Celtic chairman Brian Quinn has branded Stilian Petrov's wish to turn his back on the Parkhead club as 'bizarre'.
The midfielder signed a four-year contract in January but then expressed a desire to leave the Hoops, a wish he has reiterated in recent weeks.
Portsmouth and Fulham are both interested in the Bulgaria international but have failed to meet Celtic's valuation of their disgruntled player, thought to be around the £5million mark.
Petrov cranked up the tension between him and the Bank of Scotland Premier League champions during their pre-season trip to America by questioning the Hoops' ability to hold on to their title this season.
However, Quinn told the Sunday Times: 'I'm a bit old-fashioned. I think if you sign a contract you should stick to it.
'We had a long negotiation, and it wasn't a snap judgement by either party.
'Petrov committed himself to four more years at Celtic.
'I find it bizarre he's now taken the position he must leave.
'He's under contract to Celtic and we would only part with him if we had an offer that meets our valuation of the player.
'There's some way to go between the offers we've received and our valuation.
'We think he should buckle down and get on with it.'