don't think that madrid thing is done already. I would be highly surprised if so. i think IF he should move, we COULD have chances to get him (if we leave aside that CR doesn't want a playmaker).
BUT:
First of all, i don't think bremen will sell him. Hell he's 22, has a contract for a few more years, he's one of the best players in bundesliga, he's one of the best players in the world on his position. Even if they should get 30 mio, they won't find anyone to replace him. It's as simple as that. ATM they don't have any advantage in selling him. In 1 or 2 years, when his contract is starting to run out, and he will still have proven his worth, he can be sold for the same price, if not higher. So again, werder does not have any benefit from selling him now.
The only thing that could be FOR a transfer is his recent injury-problem. Should things complicate themselves, and should diego only be fit for half a season, then Werder management could think about selling him. They try to "replace" him already with new youngster (this Özil e.g.) but diego's footsteps are pretty big.
Now they are even risking CL-Qualification, which could of course be an argument FOR a transfer of diego. But this will happen only if he himself expresses the wish to go, Werder won't sell him that easily.