Monday's move comes less than a month after Sporting's squad was attacked by around 50 hooded supporters following a training session, reports Xinhua news agency.
There were conflicting reports in Spanish and Portuguese media about whether the decision was related to the violence or a contract dispute with club officials.
Last week, goalkeeper Rui Patricio and winger Daniel Podence also asked to be released by the club, accusing president Bruno de Carvalho of inciting violence among fans.
Sporting finished third in Portugal's first division standings last season, missing direct entry to the 2018-19 Champions League.
Portugal will begin their World Cup campaign against Spain on March 15 and will also meet Morocco and Iran in Group B.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
