Spence Jr, 31, boasts a 27-fight unbeaten record going into the clash with the former WBA (Super) welterweight champion at the T-Mobile Arena in Las Vegas.
But Lee believes that the 5 ft 9 in Texas native may have bitten off more than he can chew ahead of their showdown this summer - despite 21 of his victories coming by way of knockout.
“I give Spence the edge because of the natural size and the youth," the 37-year-old told The DAZN Boxing Show. "You just can't write off Pacquiao.
"People said the same both times and as a more methodical guy, very steady. Manny is very strong. Pacquiao is obviously explosive and fast and has a bouncing rhythm. It could be a classic, could be a war.
“I don't want to go against Pacquiao. So I'm gonna go Pacquiao, and one of the greatest submits his legacy as one of the greatest of all time.”
Spence Jr, 31, knocked out Kell Brook in eleven rounds in May 2017 to win the IBF welterweight title and make it eight stoppages in a row.
"I think they both have real challenges," he said. "Manny is Superman.
"Manny starts moving in the all-time pound-for-pound thing dramatically, not that he isn’t already a pound-for-pound guy, but he moves up dramatically if Manny is able to beat at this stage in his career Errol Spence, who I personally at the moment think is the finest welterweight until proven otherwise.
“I’d say that Pacquiao has a very daunting task, but he is one of the greatest of all time. But he's also, what is he, 39 or 40? He’s around that neighbourhood.
"I think Crawford is going to be a bigger favourite. I don’t even know. I think they're both fights with risk. I'm interested in both fights even though Spence is the favourite and Crawford is obviously the favourite. I think Manny has the more daunting task."News Now - Sport News