Word is Bruno Soriano has suffered an "undisplaced fracture of the fibula". (Undisplaced means there is basically a crack in the bone; the fibula is the small leg bone that makes up the "bump" on the outside of your ankle).
Standard treatment for this is basically immobilzation--he'll be put in a leg cast which he has to wear for 4 to 6 weeks. How long after that he can return to training is unclear, and if there is any involvement of ankle ligaments then the immobilization period might be longer.
Best case, he's available to play in mid-April; worst case, he's shut down for the season. This is a huge loss for us; as Javi Mata said, he is our Messi.
Get well soon Captain!