Some players just don't fit at certain clubs, Berbatov is an exceptional talent (remember him in the LC at Eastlands, Spuds down to 10 men and he ran the show - as good a performance from an away player as I've seen for years). Forlan was the same, great player but couldn't hack it with the reds.
Conversely, they've had players of limited ability over the years that have performed very well for them. Look at Phil Neville, the luckiest footballer in the world, hard working and loyal but hardly a world beater yet he's got more England caps and medals than you could shake a stick at!