P.Jones is out for the season and N'Zonzi hasn't played due to injury yet. Think JJ and N'Zonzi can play in the middle together.
Also JJ has played for the German full side 3 times before switching allegience to USA (father was American). Schalke are letting him go because he fell out with Felix Magath, the manager, forcing him to play in the reserves.
Excellent signing for Rovers, maybe get him full time if he does well. Who knows?