Martin Cranie is almost certain to follow flatmate Leon Best out of St Mary's.
Saints have confirmed that Cranie, 20, has rejected a new one-year contract offered to him.
And the Echo understands he could well be snapped up by another Championship club following his impressive performance in the play-off semi-final loss to Derby.
Cranie turned in a great display at centre half at Pride Park in his first league start of the season under George Burley.
Cranie's only previous Championship appearance of 2006/07 was as a late sub in the first home match of the campaign against Coventry.
He played in Burley's first four games in charge, but only managed one more league appearance in 2005/06 - against Leicester on the last day of the season.
And it is believed Cranie, who spent two loan spells with his home-town club Yeovil last season, has grown restless with a lack of senior opportunties at Saints.
Cranie has made just 25 first-team appearances for Saints since Paul Sturrock threw him in for his debut as a 17-year-old at Chelsea in May 2004.
A Saints statement said: "Martin Cranie has rejected the club's offer of a new deal.
"Unless there is a sudden change of heart he will become a free agent at the end of the month and be clear to choose which club he joins.
"Saints would be entitled to compensation for his training and development from whichever club he joins".
Burley told the Echo last month that he was hopeful Cranie's experience at Derby would persuade him to stay at Saints.
The boss said: "There's no doubt the boy wanted first team football and that's the bottom line.
"We loaned him out and he came back and played the last game and we offered him a new contract.
"I hope playing the last game would show that he has got a future at the club and we're hoping he will sign.
"He has versatility. He's still young and there's competition for places, you need a good squad and he's developing and needs to earn the right for a regular game".
Cranie, though, seems to have decided his future lies away from Saints.
That's despite the club being short on defenders at present.
Gareth Bale has already been sold and Chris Baird could follow him after handing in a transfer request.
Darren Powell has been plagued by injuries in his time at St Mary's while two other centre halves, Michael Svensson and Sean Rudd, were released last month.
If Baird was to go, that would only leave veteran Claus Lundekvam, Powell and Pele as first team squad members with central defensive experience.
Meanwhile, Leon Best has yet to publicly turn down his new contract offer.
But it is believed he, too, will reject any new deal offered by Saints and is being chased by hometown club Nottingham Forest and Coventry.
Like Cranie, Saints would be entitled to a fee - which is also likely to go to a tribunal - if Best decides to leave.
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