Batting in the ninth inning of Oakland's 5-2 victory over the Twins, Ellis took an inside pitch off his right index finger. He finished the at-bat, striking out against Minnesota closer Joe Nathan, but he was replaced on defense by D'Angelo Jimenez for the bottom of the inning and taken for X-rays immediately after the game.
The A's later released a brief statement saying that the X-rays revealed a fracture, adding that the athletic "training staff will have a further update [Thursday] during workout day at McAfee Coliseum."
Oakland's top backup infielder going into the season was Marco Scutaro, who is already filling in for shortstop Bobby Crosby, who has been out since Aug. 22 with a lower back strain. The No. 2 backup on Opening Day was Antonio Perez, who is out for the ALDS with a fractured right index finger of his own, suffered in the final week of the regular season.
That leaves Jimenez, who was signed to a Minor League contract after getting released by the Rangers in June, as the probable fill-in should Ellis be ruled out of the ALDS, too.
Where Jimenez will play is unknown. Scutaro's best position is second base, and A's manager Ken Macha gave Jimenez three consecutive starts at shortstop for evaluation purposes after the A's clinched the AL West title.
If Jimenez moves into the starting lineup, the A's will be seriously handicapped should another middle infielder go down. Teams can't replace injured players in the middle of a playoff series.
Utility man Hiram Bocachica, who made the playoff roster only because of Perez's injury, spent most of the year at Triple-A Sacramento, where he was in the outfield or at designated hitter for 70 of his 77 games, and at third base for six of the other seven. The only other backups with infield experience are catcher Adam Melhuse, who made his first career start at third base Sept. 29, and first baseman Dan Johnson.
Teams can replace injured players between series, but only with players on their 40-man roster. There are no other middle infielders on the A's 40-man roster, but Crosby has been hopeful about returning in time for the AL Championship Series should the A's make it that far.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.