Lane Garrison's Ex -- He Needs Professional Help!

Lane Garrison's ex-GF Ashley Mattingly -- whom he allegedly hit in the face during a nuclear argument last month -- tells TMZ, the former "Prison Break" actor has a serious problem ... and he needs treatment, stat.

Ashley was out in L.A. Wednesday, and we asked what advice she had for her ex -- who's still sitting in jail following the April 22nd domestic violence arrest -- and she responded, "Rehab."

It's unclear if Ashley believes Lane needs rehab for substance abuse or anger problems ... or both.

As for Lane's accusations Ashley had sent him incriminating text messages -- Ashley says Lane's full of crap ... no such texts exist.