Ellis started each of the club's first seven games before missing the next seven with the injury, which was aggravated in his first at-bat during Tuesday's 7-3 loss to the visiting Yankees. He went 0-for-2 before leaving the game in the sixth inning, and manager Bob Geren said a recent MRI showed tendinitis near the back of Ellis' hamstring.
"It's not any worse than it was before the game," Ellis said on Wednesday as his teammates prepared to take on New York for the second game of a three-game set. "In my first at-bat swinging, I knew it probably wasn't ready to play. ... A big league fastball is a little different, and it just grabbed."
On Tuesday, the 32-year-old Ellis -- undergoing his fifth career stint on the DL -- was making his first start since April 11 at Anaheim, where he initially experienced the hamstring soreness while running to first base on a grounder. He also missed a handful of Spring Training games with tightness in both hamstrings.
At the onset of his injury, Ellis and the A's believed the veteran infielder would miss only a couple of games before returning and hoping to maintain his .323 average. However, a couple of days turned into a week, and now Ellis knows that wasn't even enough time.
"I just kind of wasted seven days, but I wouldn't have known unless I tested it in a game," Ellis said. "It's definitely frustrating, though.
"I was feeling fine taking batting practice and ground balls. I felt really good at the start of the season, and I'll get back to normal and totally healthy."
Ellis' absence on the roster cleared a spot for the return of Landon Powell, who hit .162 in 20 games with the A's during Spring Training before learning the rather surprising news that he would start the season at Triple-A Sacramento while the club relied on Jake Fox as its backup catcher behind Kurt Suzuki.
At the time of his promotion on Wednesday, the 28-year-old Powell was batting .217 with a home run and two RBIs in six games with Sacramento, having appeared in three games at catcher, two at designated hitter and one at first base.
"It gives me a little more flexibility with how I use Jake Fox," Geren said. "He did a nice job for us last year."
Powell spent all of last season on the A's roster but appeared in just 46 games in place of Suzuki, hitting .229 with seven homers and 30 RBIs in his first full season in the big leagues. Right now, the catcher's presence will simply give the A's plenty of time to get Ellis back up to full speed.
"It doesn't look real bad," Geren said, "so we're happy about that. We'll get him healthy and keep our bench at full strength."
Jane Lee is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.