The Patriots lost a valuable member of their offense when Wes Welker agreed to a free agent contract with the Denver Broncos on Wednesday.

The move ends Welker’s six-year tenure in New England, during which he became the most productive receiver in franchise history.

Welker, 31, agreed to a two-year deal worth $12 million, according to reports.

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The move is a surprise, as the likelihood of Welker staying with the Patriots was thought to be strong. But nothing is a given at this time of year in the NFL, especially considering the negotiating history between the Patriots and Welker.

Now, instead of working with quarterback Tom Brady, Welker will be catching passes from Brady’s longtime rival, Peyton Manning. The deal infuses even more potentcy into the offense of the Broncos, who went 13-3 last season, and adds a new dimension into the rivalry between Manning and the Patriots.

Last year, it looked as though there was a long-term deal in place between New England and Welker after the receiver signed his one-year tender via the franchise tag. But it fell apart, and things couldn’t be finalized before the league’s July deadline.

So Welker played 2012 under his one-year franchise deal, earning $9.515 million for the season — more than half of what he’d made in his first five seasons with New England.

At the time he signed his tender last year, Welker tweeted that he was taking a “#leapoffaith,” but his gesture of dedication to his team apparently was not rewarded.

In 2011, New England offered Welker a fully guaranteed, two-year, $16 million contract, which he declined.

Losing Welker will be a blow for Brady, who always looked to him on the field and counts him as one of his close friends off it. The durable slot receiver was almost always open when Brady needed a place to throw after a play didn’t go as planned.

Brady signed a contract extension last month that could keep him in Foxborough until he’s 40; there was speculation that the quarterback had asked that Welker be taken care of as part of the deal, but team owner Robert Kraft shot down that notion in an interview with Sports Illustrated.

The Patriots learned last season how valuable Welker is. When other pass catchers, including Aaron Hernandez, Julian Edelman, and Rob Gronkowski, were injured, Welker remained Mr. Dependable, and kept his feelings to himself even when it was evident early in the season that he wasn’t intended to be a big part of the offense.

While first Hernandez and then Edelman and Gronkowski went down, Welker remained in the lineup. He wound up finishing tied for second in the league in receptions with 118, and even picked up the slack as the team’s punt returner after Edelman was injured.

Now the Patriots will have to find ways to replace Welker’s production.

He became the franchise’s career leader in receptions in only six seasons, recording at least 111 catches in all but one of his years with the Patriots (2010), and totaling 672 in that time.

DSespite being a receiver who fearlessly goes across the middle despite his 5-9, 190-pound frame, Welker missed just three of 96 regular-season games in New England. Edelman, at one point Welker’s presumed successor, has lost 16 games to injury in four seasons.

Welker did sustain a serious knee injury in Week 17 in 2009 and missed the subsequent playoff game. However, he returned for the season opener in 2010.