As i said in the other thread. The further up in weight Manny goes, the more of a disadvantage he faces. Roach says 142, but he might squeeze in an allowance for 143 for a larger guy.
Roach is trying to protect Manny from Facing a guy that would be 160lbs+ on fight night. It is totally reasonable to look out for the best interest of his fighter.
There is no obligation for Manny to face Welterweights at this point, but they are willing to meet them part way.
I think what Roach is doing is eliminating the larger welters immediately. He is leaving the door open for Cotto, JMM and Mayweather though.
Cotto and Floyd should be able to make at least 143. I think he considers Mosley simply too big for Manny, which is kinda true.