I wouldn't go as far as call him as unknown, he just hasn't played in the NFL, really. He was a first round draft pick(32nd) in 2004, but he was injured in the first game of the year, and done for the whole season. He did get the start along side Daniel Graham, and had 2 catches for 16 yards, before going down.
He was highly regarded coming out of college, He was regarded as "One of the premier tight ends in the country after a stellar junior campaign, Watson was hampered most of his final season with a nagging ankle injury … Fine athlete with tremendous quickness, boasting the fastest 40-yard dash (4.44) of any tight end in college football".
The fact that he will have to fight all those other guys in NE uniforms for catches I really can't see him being a Gates this year either. Good, yes. Gates, no. With having Watson and Graham, I think they will have a lot of 2 TE sets this year. How do you sit one of those guys???