PHOENIX -- D-backs left fielder Conor Jackson was out of the lineup Friday night against the Reds due to a sore right shoulder.

Jackson doesn't know how he injured the shoulder. It could have been on a throw or a swing in San Francisco on Wednesday. He had an MRI on Friday, but the team did not immediately know the results, or how long he will be out of the lineup.

Jackson went to see team doctors for treatment on the shoulder on the D-backs' off-day on Thursday, but he didn't feel any better on Friday.

"I think he started feeling it toward the end of the last game in San Francisco and kind of woke up yesterday with some significant soreness," D-backs manager Bob Melvin said.

"He's still feeling it today. Yesterday it was pretty significant for him. I think it surprised him the way he woke up, so he came in and got some treatment yesterday."

Jackson has been one of the D-backs' most consistent hitters in 2008, batting .294 with 12 homers and 72 RBIs in 133 games.

Replacing Jackson in the field was Adam Dunn, who made his second start of the season in left field -- his natural position -- with the D-backs.