The 32-year-old striker has netted 18 goals for the Latics this term but has scored just three times since transfer speculation linked him with a move away from the club at the end of January, and boss Joe Royle believes the decision was in the club's best interests.
"This is not a sign that we have given up on trying to qualify for the play-offs," Royle told the Oldham Chronicle. "But we have not been scoring with Lee in the side, which is why I wanted to try something different."
Chief executive Alan Hardy added: "Lee's form since January, when there was transfer speculation about his future, has disappointed both John Sheridan and Joe Royle.
"Lee was not included in the squad for the league game at Leyton Orient as the manager wanted to look at younger strikers.
"If Joe was not going to use the player for the rest of the season, there is no point him sitting here doing nothing."