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PHOENIX -- D-backs first baseman Christian Walker was not sure exactly when he strained his left oblique, an ailment that landed him on the 10-day injured list on Tuesday.

Walker felt a little tightness getting ready for Monday's 9-8 win over the Nationals, again when he reached to his left for a grounder in the top of the first inning and a little more when he tried to check his swing in his second-inning at-bat. He then finally realized in the top of the third -- after making a catch while running down the right-field line and quickly throwing to second -- that he needed to let one of the trainers know.

“I'll play through most things,” Walker said. “But something was just not letting me trust it. I was being very aware of it in the game, and when it's distracting like that and I can’t focus on the game and compartmentalize it, something tells me that I got to do something about it.”

The results of an MRI taken Tuesday morning showed a low-grade strain. Walker thinks he could possibly return in as soon as three weeks.

But oblique injuries can be tricky, so time will tell.

“Waking up [Tuesday], it was much more sore,” Walker said. “But still, all things considered, it could have been way worse, and hopefully it's only a few weeks. Feeling fortunate that it was less than what it felt like [Monday night]. So all things considered, it’s OK.”

D-backs manager Torey Lovullo said he felt partially to blame for what happened to Walker because he has played him so often this year. Walker appeared in each of Arizona's first 107 games, and he started 105 of them.

“I feel like I'm responsible for it,” Lovullo said. “I asked him to go hard, and he did. He said it had nothing to do with the intensity and the amount of innings that he was playing. He tried to make me feel better, but he can't convince me of that.”

Walker, who detests off-days, reiterated his feelings to the media after his talk with Lovullo.

“I think honestly, up until this point, this is the best my body has felt in my career at this point in the year -- strength-wise, agility-wise, recovery-wise,” Walker said. “I felt like I was really thriving, and I'm just chalking it up to some unfortunate luck.”

After the Walker injury, GM Mike Hazen acquired first baseman/DH Josh Bell from the Marlins.

Lovullo wasn’t sure when Bell was going to report to Arizona, but in the meantime, the team recalled first baseman Pavin Smith from Triple-A Reno.