While he might not officially get listed as a tight end, I think his involvement with the offense will look like it. Never known as a speedster, beating cornerbacks down the sideline isn't really his strong suit. While he is very capable of playing the slot, that position is currently taken by one of the best. Also in New England that role is typically held by smaller players who are gluttons for punishment.
Sanu is known for his strength. The strength factor comes into play in two important parts of playing tight end. Sanu can block very well and has shown that ability while in a Patriots uniform. He also has very strong, reliable hands. Those hands make him a dependable target in the middle of the field. The fact that they are huge targets at 10 1/8" doesn't hurt either.
Sanu isn't going to make a bunch of moves to get open, but he does use his quickness, smarts, and strength to get position on defenders.