Barcelona will not be able to present Luis Suarez to the press and public when his transfer from Liverpool is complete.
Southampton boss Ronald Koeman says he needs time to evaluate the contract situations of Jack Cork and Jose Fonte.
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.