So far, I've kept him on my second unit with Magic at PG. That unit runs either fast break, or Magic is just wrecking the opposing PG in the paint. If I switched Manu to my first unit, he'd be paired with PD West, and I don't know that I've used a better shooter in this game than West. I'll definitely try that out.
I find it interesting that you're using Emerald Manu and shilling for the Diamond. All along, I used the Emerald Parker and shilled for the Diamond. I like Gail and all, but I think if I had it to do over, I would have taken Tony just because I love using him. He'd be my third string PG, but I would look forward to using him.
So my advice to you is to finish the set for Manu ASAP. I got Sapphire Eddie Jones for my challenge squad and have loved using him so much that he's my 3rd string wing (I have a starting five, a backup five, and then a PG, C and a wing as my last three in case someone needs a rest) on my best squad now. After taking a break from MyTeam, I've come back realizing that it's all about enjoying the guys you're using moreso than using the best cards you have.