Walker (7-3, 2.50) started New York's 13-4 win over Pittsburgh on Friday night and was informed after the game by manager Luis Rojas that he'd be replacing teammate Jacob deGrom, who is skipping the All-Star Game in Denver.
Peralta is pitching Saturday against the Cincinnati Reds. He learned Friday night that he's the All-Star replacement for teammate Brandon Woodruff. Since Woodruff is pitching Sunday against the Reds, he wouldn't have been able to pitch in Tuesday's All-Star Game.
The 25-year-old Peralta has earned his first All-Star appearance by going 7-3 with a 2.23 ERA.
"It feels great, man,'' Peralta said. "I was waiting for this moment. It's a dream come true.''