When New York Yankees pitcher Masahiro Tanaka was diagnosed with a tear in his right elbow ligament earlier this month, the damage to his team was obvious. With a 12-4 record and 2.51 earned-run average, Tanaka had been easily the Yankees' most valuable pitcher. But the loss of Tanaka at least until September also comes as a blow to fans trying to unload their tickets on the secondary market.

For the nine home games started by Tanaka this season, the number of Yankees tickets sold on StubHub, the popular online marketplace,...