Joe Theismann sustained one of the most devastating injuries in NFL history ... and now the former star QB wants to help Dak Prescott recover from his gruesome ankle injury.
TMZ Sports talked to the former Washington signal-caller about 27-year-old Prescott's future ... after the Cowboys quarterback suffered a compound fracture and dislocation of his right ankle during Sunday's game against the NY Giants.
"I felt so bad for [Prescott]. You hate to see anybody get hurt like that. Even though the team wasn't doing real well, Dak was having an incredible year, and I just pray that he's gonna come back.