The 21-year-old winger has a sole first-team appearance to his name after featuring during last season's 5-1 League Cup win over Barnsley.
Wales Under-21 international Isgrove went on to enjoy a successful loan spell with Peterborough United from March until the end of the 2013-14 campaign, tasting Wembley glory as Darren Ferguson's side overcame Chesterfield in the Johnstone’s Paint Trophy final.
Southampton will begin the new Premier League season under new manager Ronald Koeman following Mauricio Pochettino's decision to join Tottenham.
Three key players have also departed, with left-back Luke Shaw moving to Manchester United after his England FIFA World Cup colleagues Rickie Lambert and Adam Lallana secured switches to Liverpool.
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