It's possible. I don't know what the reason is, but I know by watching his film that he didn't take the vertical shots necessary even when the pre-snap coveraged dictated that he go to his alert read. He became more of system quarterback as a Senior and operated strict to the system and the progressions. He became more of a "paint by numbers" quarterback and developed a game manager approach as opposed to his Junior season.
The emergence of Swope in the slot this past season was a direct reflection of it.
I think Swope is pretty good player... lot bigger than Welker though.
I have no idea why people think Tannehill's supporting cast wasn't a strong one. Cyrus Gray and Jeff Fuller will probably both be selected in the top 150 or so picks of this draft. Christine Michael, Ryan Swope, and Uzoma Nwachukwu will all be drafted next year. Probably pretty high for Christine Michael and Ryan Swope. And his two offensive tackles, Luke Joeckel and Jake Matthews are both potential 1st round picks next year if they declare early.