Off the top of my head I'm not positive about the protocols of the daytime networks (such as CBS in this case) being able to 'protect' a game, but if I'm not mistaken they usually get to keep one game from being flexed each week. The Pats-Broncos would certainly be the premier late afternoon game for CBS, and they own the doubleheader that week.
I guess it would come down to how much influence the NFL - who supposedly has the final say in these decisions - has over CBS. Boston and Denver are much bigger television markets than Baltimore and San Diego, and both have bigger fan bases. That would appeal to the NFL and NBC to move the Pats game to prime time, but at the same time that is also why CBS would be reluctant to give this game up.
The other thing to consider is that the NFL might try to convince CBS that the Eagles -Jets game would give them comparable ratings, as those are two of the largest television markets in the US - plus they would get the Baltimore - San Diego game back in return as an alternative.
If I had to guess right now then I would say CBS holds on to the Pats - Broncos game, and it does not get flexed to Sunday night. However, if the Chargers lose at Jacksonville? Look for a change.