Manager Torey Lovullo announced his choice Thursday at Dodger Stadium for the game Friday night against Los Angeles' Clayton Kershaw. Lovullo told Walker of his decision only a couple of hours before announcing it.
"I was ready," said Walker, who wasn't on the wild-card roster. "I was prepared. They told me to be ready for either Game 1 or Game 2."
Walker is stepping up to make his playoff debut after presumptive starter Robbie Ray was forced to throw 2 1/3 innings in the Diamondbacks' wild 11-8 victory over Colorado on Wednesday night. Ray had to come on in relief after ace Zack Greinke couldn't get through four innings against the Rockies.
Walker was in the clubhouse, pacing back and forth while the Diamondbacks held off the Rockies.
"It was a wild one, so I'm still trying to recover from that," Walker said with a laugh, recalling Arizona's celebrations. "But I'm ready. I've done my research. I've faced them a couple of times this year, so they've seen me, too. I think it should be a pretty good matchup."
Walker went 9-9 with a 3.49 ERA in 28 starts during his first season with Arizona. The right-hander has been a solid rotation starter for the Diamondbacks, who acquired him last November in a trade with Seattle including Jean Segura.
Lovullo didn't announce a starter for Game 2 on Saturday. Ray says he feels fresh after his relief appearance and would be glad to pitch Game 2.
Greinke is expected to pitch Game 3 in Arizona on Monday. He spent three seasons with the Dodgers, winning 51 games before taking that mammoth $206.5 million contract with the Diamondbacks.
Rich Hill is slated to start Game 2 for the Dodgers. Yu Darvish is expected to pitch Game 3, and Alex Wood will pitch Game 4 if necessary, Dodgers manager Dave Roberts said Thursday.
Hyun-Jin Ryu won't be on Los Angeles' division series roster because he isn't comfortable in a relief role, Roberts said.
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