At least he need a rematch against Ugas and prove if he's have a power to fight against Spence or Crawford, after that there's will be a lot hype if Pacquiao could beat them.
I believe he can match either Spence of Crawford with proper training, but he can start with a rematch against Ugas, I'm pretty sure Ugas would not say no because when he was asked after the fight against Pacman, he was open for a rematch, so Pacman could start on that if he is planning to go off of retirement.
I would stir away from Ugas, if I'm Manny and if he wanted to make a comeback. Just choose someone in the 147 lbs that he can likely beat or will take a dive just to fight money and the money that he is going to earn.
But for a Ugas or Spence/Crawford fight, it will be a suicide for him to have his comeback fight. Those 3 boxers will be too big and powerful for Manny at this point.