Aston Villa boss Paul Lambert says striker Darren Bent is not for sale ahead of the close of the transfer window.
It is understood Saphir Taider is set to leave Southampton just three weeks after arriving on a season-long loan.
Hackett, 29, made 145 appearances for the Lions, scoring five goals, since joining from Hearts in 2006.
However, he slipped out of the first-team picture this season and ended the campaign on loan at Wycombe.
Former Leicester and Watford full-back Stewart, 30, endured an injury-hit season since he signed last summer and managed just seven appearances.
Youngsters Jake Gallagher and Conor McLaren have also been released.