Manu is our backup SF at this point so unless there is a better scrap heap defender that's where he'll be (probably subbing in at SG until Kawhi comes out and Belli comes in).
Belli is what he is on defense, he's going to be on the court anyways so it doesn't matter for the purposes of your point.
So is Cojo really gonna be a difference maker on D? I am legitimately asking, can you show me some stats or replays that suggest he is anything more than an adequate defender? His DRtg of 104 isn't stellar, and it isn't leaps and bounds above Patty's 107. His reputation for defense seems to me to stem from the lack of any adequate defenders at backup PG these last few years (see: Neal, Gary).
Let's assume Mills is at least able to stay in front of his man and not make stupid obvious mistakes leading to open shots like Neal did so much. Is CoJo, maybe one or two notches above Mills at defense but with no contribution save not messing up on offense, really a better option?
I think if you cloned Ayres, Diaw, Manu, and Belli, and made our bench play against itself with Cojo opposing Mills at point, Mills' team would win at least 3/4 of the time.
Even if we assume Cojo is a good B+ defender, he's going to be shutting down whom exactly? The Norris Cole's and CJ Watson's of the league? Patrick Beverly?