PHOENIX -- D-backs first baseman Christian Walker was forced to leave Monday night’s game against the Nationals after the bottom of the third inning with what the team says was left oblique tightness.

It’s not known at this time what caused the tightness. Walker made a nice catch on a foul ball down the right-field line in the top of the third and had a ball go off his glove while reaching for a grounder to his right in the first.

The D-backs ended up winning the game in dramatic fashion, 9-8, on a pinch-hit, walk-off two-run home run by Corbin Carroll in the bottom of the ninth that capped a seven-run comeback in the final three frames.

Walker is as close to an irreplaceable player as the D-backs have. There is no true first baseman on the roster, with Kevin Newman, a shortstop, taking over for him when he left the game.

Outfielder Lourdes Gurriel Jr. has played 25 games in his Major League career at first base, but he has come in as a replacement in just two games since joining the D-backs before last season.

Walker, meanwhile, has been a big contributor for the D-backs on both sides of the ball.

Last year, he won his second straight Rawlings Gold Glove Award and offensively he leads the D-backs with 71 RBIs and shares the team lead with 23 home runs.