Tanglewood's quarterback has been sidelined for the season. It was announced on July 8 that James Levine, the musical director of the Boston Symphony Orchestra, will undergo surgery this week to remove a kidney because of a cyst causing pressure and discomfort. The recuperation period is six weeks and he is expected to make a complete recovery. It's hard to measure Levine's marquee value at a venue as famous as Tanglewood, but over the past four summers the charismatic conductor has become synonymous with the festival itself. The big question is: What conductors can be found on such short notice to replace him for his nine remaining programs? Will Levine's absence change your Tanglewood plans for this summer?