T.J. Watt's now the most expensive non-quarterback in the history of the NFL -- and James Harrison tells TMZ Sports the guy's worth every penny.
The Steelers legend said after Watt inked a new three-year, $123 million contract with Pittsburgh this week he had no problem with the big price tag -- explaining the star pass rusher is simply "the best defensive player in the league."
"All you got to do is look at his body of work," Harrison said.