I've always felt that the worst match-up for Machida is someone who can catch him coming in.
One thing Machida does is dart in on his opponents very quickly, look to land some strikes, and then dart back out. While he is very quick, he does leave himself open when he's coming in, his great timing with his bursts and just his natural speed are what he relies on to avoid getting caught, but he is still very much an open target for someone who is good at catching their opponents moving forward.
This forward blitz attack is what helped him land shots on Jon Jones:
Look at how beautiful the timing is on that, he comes forward as soon as Jones takes his leg off the ground and lands a solid left hand. Despite the beautiful timing and speed, notice how open Lyoto's chin is.
That's how Jones ultimately caught Machida in the stand-up:
Jones has great timing himself and was waiting for Machida to come forward.
If a guy like Jon Jones (who's boxing has never been anything to write home about) can do this to Machida, then someone like Anderson Silva is potentially a nightmare for Machida, because catching an opponent with backstep counters is pretty much his bread and butter.
It's hard to say for sure, because they are both brilliant strikers and very tough to figure out (not to mention that they know each other's game very well), but I could see Anderson finishing Machida.