LaRoche hurt the oblique diving for a ground ball against the Diamondbacks on Wednesday, and started feeling pain during the latter part of that game. He tried hitting off a tee Thursday, and the oblique felt a lot tighter that it did the previous day.
"I hope we caught it early enough to where we can knock it out. Obviously, the timing couldn't be any worse," LaRoche said. "It's not a severe tear, strain or whatever you want to call it, according to our doctors."
LaRoche is the second player on the Nationals to have an oblique injury in less than a week. Infielder/outfielder Mark DeRosa was placed on the disabled list Sunday because of a left oblique strain. DeRosa cannot do any baseball activities for about a week to 10 days.
With LaRoche out of the lineup, Chad Tracy was inserted into the lineup at first, batting fifth. Tracy went 0-for-2 with a walk in the 2-1 win over Arizona.