According to manager Bob Geren, the A's second baseman should not be out longer than two days.
"We're not really looking at it as anything more than a day-to-day thing," Geren said as the A's prepared to take on the host Mariners for the first of a three-game set. "It's more precaution."
Ellis, who was slotted in the designated hitter's spot Sunday afternoon against the Angels, suffered the injury in the eighth inning while running to first on an infield ground ball that resulted in a Brandon Wood throwing error. He stayed in the game and reached base in the ninth on a single to left field but was immediately relieved by pinch-runner Travis Buck.
The 32-year-old Ellis, who experienced hamstring tightness for a handful of days in Spring Training, entered Monday with the club's second-highest batting average (.345) next to Daric Barton, who is hitting .381.
Adam Rosales will fill second-base duties while Ellis recovers, and he'll carry with him an impressive starting debut that saw him go 2-for-3 with a home run and go-ahead single on Sunday.