Rex Ryan -- I'm Changing My Tattoo ... If Mark Sanchez Keeps Sucking

Jets nightmare QB Mark Sanchez better clean up his act if he wants his jersey to stay on Rex Ryan's arm -- so says the head coach, who claimed he'll ink a NEW number onto his infamous tattoo ... if Mark doesn't start playing better.

Rex was questioned about the tat during a news conference today -- and Rex joked, "Obviously, if Sanchez doesn’t play better, that number’s changing. That’s a given."

As we reported, Rex unveiled the bizarre tattoo while on vacation in the Bahamas last week -- a tat of what appears to be his naked wife wearing ONLY a Mark Sanchez jersey.

During the news conference, Rex insisted the tat's just a coincidence -- claiming he got the ink job long before Sanchez came to the Jets.

Rex said, "It’s funny. It’s actually a tattoo I’ve had for like three years ... I’ve been married for 25 years, and through my eyes, my wife’s the most beautiful woman in the world. So that’s what it is."