Some media reports say the 24-year-old starlet quit to focus on her court-ordered rehab for drug addiction; others say Wilder wanted her replaced.
The "Mean Girls" star is expected to be in rehab in southern California until at least Jan. 3.
The actress has become more of a tabloid mainstay than box office draw since a pair of high-profile arrests in 2007 led to a drug and drunken driving case. A judge last month ordered Lohan to rehab. It was her fifth time in court since May. The ruling allowed her to avoid a fourth trip to jail.