The problem with Diego is this:
Bremens team and tactic was only focused on him and only on him.
Every player had to work for Diego, give him the ball, cover his back, and so on.
The ranking was:
Diego
Team
That's one reason why Bremen is better these days. They are no-longer dependent on him. They have Özil, Marin and Hunt who can do his work without having the whole team only focusing on them. Actually they became a TEAM.
They are way more flexible and consistent these days.
And now when no one carries his ass you can see what he's really made of.