In a stunner that already has had an effect on the Philadelphia Flyers’ offseason moves, defenseman Matt Niskanen has decided to end a quality 13-year career.

Flyers General Nanager Chuck Fletcher confirmed the retirement on Monday afternoon. A short time later, he announced the Flyers had re-signed veteran defenseman Justin Braun to a two-year contract that carries an annual $1.8 million cap hit. Braun had a $3.8 million cap hit last season.

Braun, 33, could have become an unrestricted free agent Friday.

Niskanen was part of the Flyers’ No. 1 pairing in 2019-20. A 33-year-old from Virginia, Minn., and Minnesota Duluth, he stabilized the game of his defensive partner, Ivan Provorov, and helped the Philadelphia defense improve from 29th (3.41 goals-against per game) in 2018-19 to tied for seventh this season (2.77 goals-against per game).

Fletcher said Niskanen helped change the Flyers’ culture with his veteran presence, and that he played a major role in the team’s on-ice progress.

“He won’t be an easy player to replace,” Fletcher said.

A day after the season ended with a 4-0 Game 7 playoff loss to the Islanders in Toronto, Niskanen told Fletcher about his intentions to retire. Fletcher, the former Wild GM, asked him to think it over before finalizing his decision.

He did, but he didn’t change his mind.

Niskanen “felt it was time,” Fletcher said, adding that being away from his family for so long during the postseason -- and the uncertainty that the pandemic has brought to next season -- impacted Niskanen’s decision. It was not a health-related decision, Fletcher added.

Niskanen had one year left on a contract that had a $5.75 million cap hit. Like many of his veteran teammates, he struggled at times in this year’s postseason, notching two points and an even rating in 15 games.

Niskanen, who won a Stanley Cup with Washington in 2018, had eight goals (tied for the second-highest total in his career), 33 points, and a plus-15 ranking in his first and only season with the Flyers. A first-round pick (28th overall) by the Stars in 2005, he played 13 seasons with Dallas, Pittsburgh, Washington and Philadelphia.