Paul Heckingbottom has shocked Darlington after deciding to shelve contract talks aimed at extending his stay at Feethams.
His current deal runs out in the summer, but before committing himself to another Quakers contract, the 24-year-old has decided to listen to any offers.
Heckingbottom would be able to leave Darlington on a Bosman free transfer which would be a massive blow to Quakers as, despite spells on the sidelines through injury, the former Sunderland defender has been a committed performer ever since arriving at Feethams in March 1999.
Manager Tommy Taylor said: "He most probably has an agent and he'll be telling him to hang on to see if he can get a deal with a bigger club elsewhere.
"I'm disappointed with the boy because I've been open with him and I just wanted to sit down with him and try and sort something out.
"There's not been any official contact from any clubs.
"But you can't blame him because he is good enough to play at a higher level."
Heckingbottom, who was team captain in the absence of Craig Liddle recently, signed from the Black Cats and has made 92 Darlington appearances, including four as substitute.
Of Tommy Taylor's first choice back-four Heckingbottom will be the only Darlington defender available at third-place Mansfield on Saturday because of injury suspension.
Liddle will serve a one-match ban, Simon Betts is out for the foreseeable future after taking a kick above the knee at Carlisle while Gary Pearson is struggling with a hamstring strain.
However, it is hoped Pearson will return for the home game with Hartlepool on Tuesday, while David Brightwell may also come back into the squad after recovering from a calf injury.
Last night's reserve game at home to Grimsby Town was postponed due to a waterlogged pitch, meaning Taylor was unable to cast his eye over a trialist from Sunderland.
Instead, the central defender will train with Darlington this morning.
Read more about the Quakers here.
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