It looks like the suspicions were correct. Here's the latest, according to Stephen J. Nesbitt of the Pittsburgh-Post Gazette:
The pathology report from Jameson Taillon's surgery last week came back positive for testicular cancer, Pirates head athletic trainer Todd Tomczyk revealed Wednesday. Taillon is scheduled to be re-evaluated by doctors and surgeons next week and map out a treatment plan.
Nesbitt notes that Taillon has rejoined the Pirates in the past few days, and that he's been cleared to partake in "light baseball activity," like playing catch. Taillon had made six starts on the season before hitting the DL, during which he compiled a 131 ERA+ and a 2.31 strikeout-to-walk ratio.
Taillon's positive test makes him the second big-league pitcher known to be battling testicular cancer, joining Chad Bettis of the Colorado Rockies. Here's hoping it won't be long before both are cancer-free and back on the mound.