I might be alone in this but I thought, at the time anyway, that Mackie being put on for Clark was a good idea. Every time Clark got the ball he seemed to stop, consider what to do which gave Hibs enough time to get into a decent defensive position, and Clark would therefore be forced to pass the ball backwards.
At least with Mackie there was someone with pace who didn't slow down every attack that came through him.
I'm not saying he's a great player but I thought that substitution made sense.