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New England Patriots tight end Rob Gronkowski has retired. He announced his decision Sunday on Instagram.

By his incredibly high standards, the 29-year-old had a down year in 2018. He caught 47 passes for 682 yards and three touchdowns in the regular season and added another 191 yards in the playoffs.

The five-time Pro Bowler also ranked sixth among qualified tight ends in DYAR (defense-adjusted yards above replacement), per Football Outsiders.

While Gronkowski's decision is somewhat unexpected given his age, it's not entirely surprising.

After the Patriots lost to the Philadelphia Eagles in Super Bowl LII, he told reporters he was going to weigh his future in the NFL and wouldn't give a firm commitment to playing in 2018.

His status remained a storyline during the offseason until he finally confirmed last April he would be suiting up for New England.

Gronkowski's potential retirement again became a topic of discussion prior to the Patriots' AFC divisional-round win over the Los Angeles Chargers. He missed three games this past year, and NFL Network's Ian Rapoport reported other factors were at play:

"While he's feeling physically good going into today's game against the Los Angeles Chargers, those who know him well say the season has taken an emotional toll on him. Perhaps strongly considering retiring last year put him in a place where it was difficult to get fired up for this season.

"Gronkowski hasn't seemed to be the same effusive personality and people close to him say everything has been a battle. They wonder how much he loves it."

While Gronkowski hasn't had a long career compared to contemporaries such as Tony Gonzalez, Jason Witten and Antonio Gates, the mileage has added up. He hadn't played a full season since his second year in 2011, and he battled major injuries to his right knee and left arm in recent seasons.

During the buildup to the Patriots' 13-3 Super Bowl LIII win over the Los Angeles Rams, Gronkowski reflected on the physical rigors of playing football, per NFL Network's Michael Giardi:

"You take hits to the thigh. You take hits to the head. Abusing your body isn't what your brain wants. When your body is abused, it can bring down your mood. You gotta be able to deal with that. I just took 50 collisions and the next day everyone wants you to be up. They want practice to be full speed. The next week, they want the game to be full speed but they don't understand what players are going thru with their bodies or their minds."

Gronkowski can walk away knowing he achieved a lot in a relatively short amount of time. In addition to his five Pro Bowls, he won three Super Bowls and earned four first-team All-Pro honors. He's also eighth all-time in playoff receptions (81), ninth in playoff receiving yards (1,163) and tied for second in postseason touchdown receptions (12).

He will be a tricky Hall of Fame case, though. He clearly boasts a strong resume but might get docked for his lack of longevity. Ultimately, the Hall of Fame voters are likely to look at his impact when he played and focus less on how his persistent injuries limited his ability to continue playing.

As much as Gronkowski has meant to the offense, the Patriots might be happy to finally see a resolution to what has become a lengthy saga. ESPN's Adam Schefter reported last September that New England nearly settled on a trade to send him to the Detroit Lions, but he scuppered the deal by again mulling retirement.

Losing the University of Arizona product will create a massive void in New England's passing game. Replacing Gronkowski's presence and production with another tight end will be next to impossible.

Still, fans will trust head coach Bill Belichick to find a way to make things work.

The Patriots' biggest problem will arise if Gronk's retirement impacts Tom Brady's choice to play in 2019.