There's no reason why it should be short term. Federer has had a mixture of huge form dips and minor form dips. However, each dip has been accentuated by stronger competition over the past 4 years. His 2008-2010 form would still have carried him to 3 out of 4 slams against the 2004 field, despite a noticeable loss of form.
I don't see any physical decline whatsoever and see no reason why he can't sustain his current form.
He doesn't move as well, he doesn't time his forehand as well as he once did (albeit it's still a great shot from time to time) and his backhand's regressed a hell of a lot.