Muhammad Ali would have been in full support of NFL players taking a knee during the national anthem ... because his ex-wife tells TMZ Sports "he would've been right there with them."

Khalilah Ali -- who was married to the boxing legend from '67 to '76 -- says it brought tears to her eyes to watch all the players kneel "in unity" last weekend ... and thinks Muhammad would've been proud to see the protest he started continuing to this day.

"Muhammad would have been right there with them, rooting them on and giving total support. Because he started it. He started this."