McEntee, one of the Saddlers' 11 summer signings, took to social media to announce he had undergone an operation having picked up the injury while playing for Ireland Under-21s earlier this month.
The 21-year-old was forced off with a dislocated shoulder during a 3-0 victory over Bosnia & Herzegovina. It is not known how long he will be out for at this stage.
McEntee posted a picture from his hospital bed on Instagram along with the message: "Operation was a success, thanks for all the messages! Road to recovery starts now."
Walsall signed McEntee on a two-year deal upon the end of his contract at Premier League side Newcastle United.
He spent last season on loan at Scottish Championship side Greenock Morton, playing 28 times as they finished seventh in the league.