Eovaldi, a Texas native who signed with the Rangers last offseason, will make his first career World Series start. The right-hander is 4-0 with a 2.42 ERA in his four starts this postseason, including the AL Wild Card Series and AL Division Series clinchers, and the must-win Game 6 of the AL Championship Series at Houston on Sunday night.
"Nate's had plenty of rest, he's ready to go," Texas manager Bruce Bochy said Wednesday.
Eovaldi, one of Texas' offseason acquisitions, has a 2.42 ERA in four starts this postseason. He won Games 2 and 6 in the series against the Astros and is unbeaten this postseason.
The Diamondbacks, who were flying to Texas the day after clinching the NL pennant in Philadelphia, are going with the right-hander Gallen, who is 2-2 with a 5.24 ERA in four games this postseason -- his first.
He started Arizona's openers in both the NLDS and NLCS.
Gallen posted a 17-9 record with a 3.47 ERA in 34 games during the regular season. He led the Diamondbacks in wins, strikeouts (220) and innings pitched (210).
He struck out 11 and allowed one run in six innings of a 6-3 victory over Texas on Aug. 22. He is 1-1 with a 3.57 ERA in three career appearances versus the Rangers.
Bochy didn't say who would start Game 2 for the Rangers, though left-hander Jordan Montgomery is 3-0 this postseason, including the Game 7 victory in Houston with 2 1/3 innings in relief on Monday night.
The 33-year-old Eovaldi is 8-3 with a 2.87 ERA in 15 career postseason games, including 10 starts.
Eovaldi's only previous World Series was with the Red Sox in 2018, when he made relief appearances in each of the first three games against the Los Angeles Dodgers. He pitched the final six innings of the 18-inning third game, when Max Muncy led off the bottom of the 18th for the Dodgers with the game-ending homer.
Reuters and The Associated Press contributed to this report.