Roku doesn't provide the content, it's merely the streaming platform. Since you have Comcast, there is an Xfinity channel for Roku devices in the channel store, and it should provide you with whatever content you subscribe to. For something such as a particular sports team, there might be local blackouts on viewing. They also might be available through the NFL channel, which requires a subscription. If you subscribe to NFL through Comcast, you might be able to use the NFL channel for Roku devices.
But as to your question, I seriously doubt you'll find any major sporting events that are "free". One way or another, you have to pay to see them. Yes, you'll need to install a channel to view them. And more than likely it will cost something. An NFL game available for free would most likely be in violation of the Roku terms of agreement with a channel provider, and that channel would soon disappear. But if you have a subscription to NFL through some other means, such as through Comcast, then you might have access through the NFL channel. I don't watch sports, so have no idea how it might work for that specific sports franchise.
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