Many players experienced a drop in performance after and incredible start to their careers. Reason does not matter.
Short list - Terry Sawchuk, Frank Mahovlich, Earl Reibel - who lead the Red Wings in scoring at the age of 24 during the 1954-55 season ahead of Howe, Lindsay, Delvecchio and Kelly,
http://www.hockey-reference.com/teams/DET/1955.html
Recently you have had Alexander Mogilny, Teemu Selanne, Jeremy Roenick, Eric Lindros, Real Cloutier amongst others that saw drastic drops in performance after promising starts.
So it is definitely realistic to tkae nothing for granted.