Michael tells TMZ ... he's been fighting skin cancer for a while, being diagnosed with squamous cell carcinoma about 4 years ago and "recently had a suspicious lesion on my hand" that prompted doctors to do a biopsy.

Things changed Tuesday night when he got a call regarding the biopsy and that he needed to get the patch removed ASAP -- and Wednesday, that's exactly what happened.

He tells us the cancer had "deep roots" and needed a lot of digging, but we're told they thankfully were able to get it all out.

Michael admits the type of cancer he has is "aggressive" and he's constantly monitoring it ... adding it likely will return at some point.

Lindsay's father posted about his new scar on Instagram Wednesday, writing, "A scar means the hurt is over, the wound is closed, you endured the pain, and God has healed you."

BTW, we're told his son, Michael Jr., was the only one who knew about his medical situation -- and Michael tells us he didn't want Lindsay to worry or affect her work, which is why she was unaware.