Yu Darvish had been expected to start Tuesday night in Arlington but was a late scratch for Texas, which can clinch at least a wild-card berth by defeating Oakland for the sixth time in eight meetings.
Just 2 1/2 hours before the game, the Rangers announced Darvish would not take the mound due to neck stiffness. Darvish, the 16-game winner who is 4-0 with a 1.46 ERA over his last five starts, will be replaced by Scott Feldman.
The Rangers (91-62) extended their AL West lead over the A's to five games with a 5-4 walkoff victory in Monday's series opener. Adrian Beltre went 3 for 5 with his 35th homer - a two-run shot in the seventh inning to tie the game - and the winning RBI single.
Josh Hamilton hit his major league-best 43rd home run after missing the previous five games due to vision problems, and Beltre has been playing through stomach pains lately.
''He's playing incredible right now,'' second baseman Ian Kinsler said of Beltre. ''Anytime you've got a guy like that behind Josh Hamilton, it's dangerous. To get Josh back in the lineup, it obviously gives our offense a little bit of a boost because they have to pitch to Josh, and if they don't pitch to Josh, then they have to face Adrian. So it's kind of pick your poison with those two guys.''
Beltre was hitting .184 against Oakland this season entering Monday.
''Every game for us is big,'' Beltre said. ''Especially facing the team right behind us. We want to clinch as early as we can."
The A's loss put a damper on their chances to win a division title for the first time since 2006, and it also trimmed their lead over the idle Los Angeles Angels to two games for the AL's second wild card.
Yoenis Cespedes and Josh Donaldson homered for Oakland (86-67), which has dropped six of eight.
''It's one game. We have to bounce back,'' said manager Bob Melvin, whose team faces the Rangers six more times over the final nine games. ''It would have been nice to start off with a win in the series, but you have to move on from it.''
Two-time defending AL champion Texas has never made three straight trips to the postseason, and now turns to Feldman (6-11, 5.19 ERA) to help make it happen Tuesday.
The right-hander came out of the bullpen Friday for the first time since the All-Star break in his most recent appearance, throwing three scoreless innings at Seattle. He had been struggling, going 0-5 with a 6.99 ERA in his last seven starts.
Feldman has split two starts against the A's this year but didn't pitch well in either one, allowing 12 runs and 16 hits in a combined 6 2-3 innings.
Oakland counters with Tommy Milone, who looks to regroup after a shaky outing.
Milone (13-10, 3.86) gave up three runs and nine hits while walking three in 4 2-3 innings before Oakland scored 10 times in the final four innings for a 12-4 win over Detroit on Thursday.
The left-hander has a 4.95 ERA over his last four starts, sandwiching a pair of rough outings around two solid performances. He's lost both of his starts versus the Rangers this season, allowing 10 runs - five earned - in 12 innings.
Beltre (two homers) and Hamilton (homer) each are 3 for 6 off Milone.