Of course he might not win another slam, he's just had serious back surgery. Murray is just stating the obvious.
I think he is just downplaying his chances
From the recent tidbits of info from the social media, he looks supremely fit and ready to hit the ground running.
Of course, competing against the uber elite in the world is a completely different ball game altogether, even Rafa took several months to ease into the game and was rusty for most of the clay season. So Andy might not be at his very best until the IW-Miami stretch or even the later parts of clay season.
Anyway, we will get to see how Andy is feeling in a couple of days