Charlie Sheen MIA on Set, Denise Sounded Alarm

Charlie Sheen's ex-wife warned people on "Two and a Half Men" that Charlie needed help badly ... sources tell TMZ.

We've learned Denise Richards contacted an exec from the show Monday morning and said she was concerned about Charlie relapsing and wanted someone to step in and convince him to go to rehab.

Sources connected with the show tell us when Charlie did not show up for work on Monday or Tuesday, it became apparent to "Two and a Half Men's" Executive Producer Chuck Lorre that Friday's filming was in jeopardy.

We're told Charlie himself realized the show, along with his freedom because of the whole bail issue, were on the line ... and that is what forced his hand.

More sources have come forward and confirmed TMZ's story ... that Charlie is the one who made the decision to go to rehab.

And something else: Our sources say at 10:30 AM yesterday a plan was in place to admit Charlie at 8:00 PM to the rehab facility. In other words, Charlie didn't get cold feet.

As we first reported, the plan is for Charlie to return to the set two weeks from Monday. If that happens, Warner Bros. can deliver the remaining 6 episodes for the season to CBS on time.