In the buildup to Chelsea signing Diego Costa, he was frequently portrayed as a sort of one-season wonder, who turned up out of nothing in 2013/14 and proceeded to maul the rest of La Liga. And in a certain sense, that's true -- he certainly wasn't Atlético Madrid's main man prior to last year.
But that's because Atlético Madrid are a striker factory. Before Costa was Radamel Falcao; before Falcao came Sergio Agüero. Diego Costa wasn't a big signing. He had to fight his way into position to take advantage of Monaco's move for Falcao, he then had to fight to earn the same sort of move that his predecessors managed. In the process, Atléti won La Liga.
Jose Mourinho -- who also confirmed that Costa had been a target since January -- is, not unreasonably, impressed with his career path.
I’ve known Diego since he was 17 and he went to Portugal. He is the kind of guy whose life was not easy, nobody gave him anything for free. He has always had to fight a lot to do things.
In Portugal he was a Braga player and it’s a good team. He then had to go on loan to a Championship team to be back at Braga. Then Atletico Madrid bought him, but after that he goes to Celta Vigo because Atlético had (Sergio) Aguero. Then he had to go to Real Vallecano because Atlético had (Radamel) Falcao. He comes back to Atlético when Falcao goes. The guy had to fight a lot to arrive there, I know his psychological profile and I think he is the kind of guy who is afraid of nothing and ready for everything.
It's impossible not to read some subtext regarding another striker here. Praising Costa's attitude towards fighting for a place in the team when he wasn't first choice, of battling through the necessary loans, of being 'afraid of nothing and ready for everything' isn't really a shot at Romelu Lukaku (although I can see how it might be taken that way), but it is a strong piece of advice. 'Watch this man, Romelu,' Mourinho seems to say. 'Learn from what he has achieved, apply yourself to the same ends. This is how to make it to the top.'
Leaving the subject of large Belgians to one side, it's also an interesting reminder that there's a second way of looking at Costa's supposed one-season-wondrousness. Yes, he only has a year of being an elite centre forward, but he's only had one chance to be an elite centre forward, and he took it with extraordinary aplomb. It's difficult to criticise him for that.