Monfils is a very dangerous first match for Murray. If he is a bit rusty (and I doubt he has played much since the US Open) he could easily lose this match.
He started training again a couple of days later. But I still wouldn't underestimate Monfils. Andy's whole draw (if it holds up) is absolutely awful. Berdych will have a much easier time getting to the final. Of course, in his case, that doesn't mean anything.