Marsh isn't soft-tossing, he averages 94 on his fastball. If you want to say they struggle against good off-speed pitching, then I'd agree, he had good off-speed stuff last night. The Red Sox decided they aren't going to throw fastballs this year, we'll see how that works out for them going forward.
PS. All teams struggle against good off-speed stuff when a pitcher is on. Marsh was on.