Gronkowski originally suffered a fracture of his left arm in a game Nov. 18, which forced him to miss five games. He returned in the season finale against the Miami Dolphins, but in a limited role.
On Sunday against the Houston Texans, Gronkowski fractured the same arm on an attempted diving catch in the first quarter.
Since the injury, speculation has been that Gronkowski was rushed back and that his injured left arm wasn't ready for contact. The source says the current injury "had nothing to do with whether the arm was ready or not."
Instead, Gronkowski's arm broke at the end of the protective plate that was implanted in his arm.
There will always be a risk to the area at the end of the plate, the source explained, because of a "collection of stress." The injury could have occurred "three years from now" in the same spot, said the source.
Gronkowski's original break, it was noted, has fully healed and was intact when surgery was performed Monday to address the new injury.
The injury will force Gronkowski to sit for the rest of the playoffs.
Gronkowski is considered one of the best tight ends in the NFL. While the Patriots do have a crop of talented receivers, including tight end Aaron Hernandez, to help replace Gronkowski, the third-year man out of Arizona has always been a game-changer for the Patriots.
Last season, Gronkowski caught 17 touchdown passes, an NFL record for a tight end. He also had 90 catches and 1,327 yards receiving.
He played in just 11 games this season because of the previous fracture. He caught 55 passes for 790 yards and 11 touchdowns. He was averaging 71.8 yards per game, which was fairly close to his 2011 average of 82.9 per game.
After Gronkowski left Sunday's game, tight end Michael Hoomanawanui was inserted into a more important role. He didn't have any catches, but Patriots coach Bill Belichick praised the play of the third-year man.
The Patriots play the Baltimore Ravens in the AFC Championship game Sunday at 6:30 p.m. ET.