MLB instituted a rule in 2010 banning starters who pitched on the Sunday preceding All-Star break from pitching in the game. That rule was modified by the 2012 CBA which allows a "Sunday pitcher" to appear for no more than one inning in an All-Star Game, if they choose to participate.
Scherzer has a 2.12 ERA with 143 strikeouts and two complete-game shutouts so far this season, and was selected to the National League team via the player's vote.
With Scherzer no longer an option, Bruce Bochy will have choose from a stable of arms that includes the Dodgers' Zack Greinke, the Pirates' Gerrit Cole and A.J. Burnett, the Mets' Jacob deGrom, the Giants' Madison Bumgarner and the Cardinals' Michael Wacha.
The 86th All-Star Game will be played at Great American Ballpark in Cincinnati on Tuesday, beginning at 7 p.m. ET.
This post was updated to include information on 2012 CBA modification.