I mean...do we really want Rudy Gay shooting pull up mid range jump shots in the playoffs. Over everyone else on our team? Shot creation is the last thing this team needed, and that is Rudy's biggest strength. This isn't a Joe Johnson situation where the team desperately needed another scorer. We have multiple players who only score.
Offensively, the biggest thing we need from this position is someone to make wide open shots. While Gay just had a career shooting (hopefully he doesn't regress at 35), we just replaced a guy who shoots significantly better. Also, if we're going to take his 3 point shooting for last season was real, we ought to point out that his mid range was bad last year. That's why he was wayyyy below league average efficiency.
Defensively, he is an upgrade over Niang I'll give you that. As long as age doesn't catchup to him. The rebounding is a nice plus too. Gay only played about 100 minutes at C last year. He can always start playing C, but it's not really something he's done before. I'm not sure what having the small 5 does for us anyways. Does it space the floor? I suppose, but the rim running C has provided so much value in our offense which has not been a problem. Does it do anything for us defensively? Not really.
If it was a perfect fit, he would provide the things we need most. Perimeter defense, defensive rotations, defensive playmaking ect. Rudy Gay might still be a nice payer, but that is not his game. His game is providing shot creation and that is not a need at all.