I think Murray will only really be mentally hit if he failed to win a Grand-slam and neither Federer or Nadal were in it.
Right now, Federer is still there, and he is favourite, there is no pressure on Murray.
at present, players are defined by how they handle federer and nadal at slams. so far, murray has been woeful - with the exception of few easy wins over rafa, that a) he matches up very well against, b)was played on hard courts, and c)was not a 'peak rafa'.
add to that fed is in his 'autumn' faze, facing him in the finals will give no comfort to murrays state of mind. on the contrary, loosing 3 finals to fed (with 2 already being straight sets defeats) - is a major, major, blow for any aspiring top-top player.
if it happens, it may very well be a defining loss or win.