Rivaldo has serious problems with van Gaal and it's only reasonable that his game has deteriorated. Try to remember him in the field in November or January. Absolutely nothing to do with what we're seeing now.
It should be obvious that if van Gaal stays, he'll leave. Don't rush to gloat though, the fact remains that he knows football, I wouldn't be at all surprised if he goes to e.g. Lazio next year and does wonders. Then you'll be sorry he left, won't you?