For Wednesday night’s Carnegie Hall season-opening gala with the Philadelphia Orchestra, the Chinese superstar was booked to play Gershwin’s Rhapsody in Blue with Chick Corea in a seldom-heard two-piano version. Then, this past spring, Lang Lang injured his left arm – he says it was by practicing Ravel’s Piano Concerto for the Left Hand too hard. So he’s bringing in a 14-year-old protégé to play the left-hand part alongside him.