Harmon is expected to start against the Carolina Panthers next week to replace Gregory, who sustained a broken thumb in the Patriots' win over the Pittsburgh Steelers.
"He's probably the smartest football player I've been around or seen," Harmon told reporters.
"He'll see something from the offense and already know what's coming," Harmon said. "I just guess it comes from experience and playing a lot of football."
The Patriots are 7-2 going into week 11.