The 43-year-old actor sparked concern after he recently dropped 30 pounds from his already lean frame for his role as an AIDS patient in the new movie 'The Dallas Buyer's Club'.
But a source close to the father-of-two - whose model wife Camilla Alves is pregnant with their third child - is now at his lowest weight.
The insider told People magazine: "Matthew starts shooting this weekend, so he's at his absolute lowest weight. In a month, he'll begin to put it back on."
The actor has flown to New Orleans to begin filming the new movie in which he plays Ron Woodruff, an AIDS patient diagnosed in the mid-1980s who seeks out experimental HIV treatment in a desperate bid to prolong his life.
Last August the 'Lincoln Lawyer' star admitted he found losing the extreme amount of weight to be a huge mental challenge but said he was determined to lose the 30 pounds required for the role.
He said: "I'm playing a guy who was sick and would have loved to have been healthier but wasn't. For me, it's more of a mental thing than a physical thing," he explained, adding that he's been eating "small amounts" of high-protein, low-carbohydrate food.
He added: "I'll get down to the weight I need to get to. I'm on my way, and it's what I need to do for the job."