When you subscribe to a cable company (Comcast, Verizon, etc.) the cable line coming into your home transmits everything they can possibly send you in an encrypted signal. Then the cable modem and set top boxes decrypt the signal based on the level of service you pay for.
A few years back the FTC stopped allowing the cable companies to encrypt any channels that are available OTA, such as the networks. If you keep Internet service from your provider but cancel the television service (which you would need to do to continue using your Roku) you *should* be able to hook the cable line into the television and use the TV's QAM tuner to continue receiving network broadcasts. In addition, some Internet companies offer ESPN3 as part of the Internet service, which includes Monday Night Football and many college football games. I'm not a baseball fan so I'm not sure about that.
So at my house I watch network games over the cable plugged directly into my television, and ESPN games through the WatchESPN app on my iPad streamed to my AppleTV. I find this gives me access to almost all of the NFL games I want and I only pay for Internet Access.