Winslow Townson/Associated Press

Rob Ninkovich's 11-year NFL career came to an end Sunday, as the New England Patriots linebacker announced his retirement at the age of 33 during a press conference.

Patriots head coach Bill Belichick confirmed the decision, saying he did so with "mixed emotions," adding he couldn't thank Ninkovich enough for his contributions to the franchise.

ESPN.com's Mike Reiss first reported the news.

There had been growing speculation about Ninkovich's future. He skipped New England's first three practices this offseason, including Saturday's first day in full pads.

Per NFL Network's Mike Garafolo, Belichick said Ninkovich's absence was for "personal reasons" but didn't elaborate.

Ninkovich addressed his situation July 19, though he didn't offer a definitive take.

"When I was younger, you see a guy who's my age or older and you're like, 'Jeez, that's so and so? The Pro Bowler?' It's kind of one of those. It's sad. ... You have to monitor yourself and make sure you don't get to that point. ... I still feel really good. That's the one key," Ninkovich said, per Mark Daniels of the Providence Journal.

After bouncing around as a part-time player with the New Orleans Saints and Miami Dolphins in his first three seasons, Ninkovich became a star in his eight seasons with the Patriots.

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Ninkovich led the Patriots in sacks twice (2012, 2014) and was a versatile defender, shifting between end and linebacker.

In 2012, Steve Palazzolo of Pro Football Focus named Ninkovich as New England's secret superstar because of his ability to do everything on defense.

"As the Patriots evolved, so did Ninkovich, and he continued his stellar play all the way to the Super Bowl," Stockwell wrote. "He's doing a job that often takes three roster spots to fill, and in a league that embraces specialists—whether pass-rushers, nickel cornerbacks or slot receivers—Ninkovich's value has been as a jack-of-all-trades."

Ninkovich's purported retirement came as a surprise because his performance hadn't dropped off and he was under contract through 2017 after signing an extension before Week 1 last season.

In September 2016, he was suspended for four games for testing positive for a banned substance.

After returning in Week 5, Ninkovich helped the Patriots win the AFC East for the eighth straight season and claim their fifth Super Bowl title with a 34-28 overtime victory against the Atlanta Falcons.

Ninkovich also won Super Bowl XLIX with New England. He started out as a nondescript fifth-round pick out of Purdue in 2006.