Joe Theismann says watching Gordon Hayward's leg snap during NBA's opening night took him right back to 1985 ... an eerie reminder of that fateful encounter with Lawrence Taylor.
Of course, LT snapped Theismann's leg live on "Monday Night Football" -- essentially ending Joe's career. It remains one of the most gruesome injuries of all time.
Theismann tells TMZ Sports he really does understand Hayward's pain -- and feels terrible every time he watches a similar injury on TV.
The good news ... Theismann says he's optimistic Hayward can recover and go on to have a strong career in the NBA -- as long as he follows two important rules.