Over the years, especially in the Premier League, we've brought in players that were doing well in their respective divisions, well enough that we decided they should come and play for us. Then, magically, they've been ****, our fans write them off and they leave with a cloud over their heads. But, by some miracle, they go to another club and they start doing well again. Granted, a few drop down to a lower level, but there are examples of players going on and rediscovering the form that led us to buy them in the first place.
Altidore, Proschwitz, Sagbo, Ince (did well in the Championship before and after, was a victim of Bruce's tactical switch to 3-5-2), Diomande, Elabdellaoui, Henriksen, Ramirez, Mbokani. All came here with decent pedigrees and records, all left with a big fat "they're ****" label on them. And that's just in recent seasons. Note that more than half the names on that list are strikers, too.
Sometimes, it just doesn't work out with some players, sometimes they don't settle, sometimes they get injured. But the ones I've listed above aren't bad players, not by any stretch and it just so happens that when they signed for us, they were crap, but they're now playing for other clubs that get more out of them.