After hitting .250 through April, Smoak has batted just .185 in the past month -- including 1-for-10 on the current trip -- as his average has dropped to .212.
"Just giving my first baseman an extra day off," manager Lloyd McClendon said. "He's got a tight quad and probably a little fatigued. Instead of one day, we'll give him two [with Thursday's off-day]."
McClendon said the quad has been bothering Smoak for "quite a while," as the big first baseman was the only Mariner to have played in each of the team's first 58 games.
"I think fatigue has something to do with that," McClendon said of Smoak's extended slump. "This will give him a chance to freshen up, and hopefully that will quicken his bat up a little."
With Smoak out and the Mariners facing left-hander Mike Minor, McClendon put rookie right fielder Stefen Romero at cleanup a day after he hit a three-run, pinch-hit homer. Kyle Seager dropped down to fifth to avoid three straight left-handers batting against a lefty starter.
"Somebody has to be there," McClendon said of his cleanup spot. "We have to fill the lineup out. There's no significance to it, trust me. Somebody has to hit four. I don't have Bonds, so it's going to be Romero."
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.