According to a report on Comcast SportsNet New England, Gronkowski has a broken forearm.
"He left the game with an arm injury," Patriots spokesman Stacey James said. "There was no update to that."
Gronkowski walked to the tunnel that heads to the locker room with about four minutes left in the Patriots' 59-24 victory over the Colts. No announcement of his injury was made during the game or for a couple hours afterward.
NFL rules state "... team personnel are responsible for reporting in-game injury information factually and accurately as soon as possible for the benefit of the network television audience and the other media covering our games."
The happy-go-lucky Gronkowski had 10 touchdown catches as a rookie and then set an NFL record for tight ends with 17 touchdowns last year, when he caught 90 passes for 1,327 yards. But he sprained his left ankle in the AFC title game and was a nonfactor in the Super Bowl, which New England lost 21-17 to the New York Giants.
He had seven catches for 137 yards on Sunday, including touchdowns of 4 and 24 yards.