He got Pompey syndrome.
Every now and again, we get a great striker who plays for us, does amazingly well, looks like a Premiership player, decides to go to a bigger club, and flops flat on his arse.
See also: Guy Whittingham, Lee Bradbury, Gerry Creaney, Deon Burton. Remember them? Thought not...
Guy Whittingham
^^^
I remember him moving from Villa to Sheffield Wednesday in exchange current Derby player Ian Taylor.
Lee Bradbury
^^^
He joined Derby on loan this season but got injured.
Deon Burton
^^^
Went between Pompey and Derby last season.
Interesting links between these players you mentioned and Derby!