David Nelson doesn’t know for certain why the Buffalo Bills decided to let him hit free agency.

The Bills might’ve wished the fourth-year receiver luck because he won’t fit in new head coach Doug Marrone’s system or because the $1.323 million qualifying offer was more than they wanted to commit.

Nelson, though, is convinced it’s because of the season-ending torn knee ligament he suffered on opening day.

“It’s tough for me because I feel in my heart – and this is just me – the only reason it’s happening is because of the injury,” Nelson told The Buffalo News in his first interview since the Bills informed him of their decision.

“I feel if I was able to play last year, things would be different 100 percent. For me that’s the reason why; they’re not sure if I’ll be the same player I’ve been.”

The Bills declined comment.

The injury cost Nelson a season of production and – a legitimate possibility – the Bills a victory or two.

Nelson, calling it “sobering to re-evaluate,” also is coping with the idea that a twisted knee, while engaged in a meaningless block late in a blowout loss to the New York Jets six months ago, has impacted two seasons.

“It’s upsetting,” Nelson said Wednesday night from his home in Dallas. “It’s already hard enough to sit out a year and not be able to be with your teammates on the field.

“Then to find out they’re not going to bring you back because – and this is my opinion – because of the uncertainty of the knee and doubts if I can be a good player is the hardest part for me.”

Nelson, an undrafted rookie in 2010, became a significant part of Buffalo’s offense two seasons ago. He caught 61 passes for 658 yards and five touchdowns. Ryan Fitzpatrick looked for him in many critical situations.

“The hardest part is the realization that we’re not going to be able to finish what we started and what we were building on,” Nelson said. “All that work we put in – individually and collectively – to bring that city something they deserve is difficult.

“That city deserves a championship. They’re dying for a playoff-caliber team.”

Bills Vice President of Football Administration Jim Overdorf called Nelson’s agent, Joel Segal, on Feb. 24 to inform him of the club’s decision. Nelson said he hasn’t spoken to the Bills or sought information since.

Nelson has been training at Athletes’ Performance in Dallas. He recently began running routes with free-agent quarterback Seneca Wallace throwing to him. Nelson said the original rehabilitation schedule estimated he would begin running routes in April.

Nelson described his biggest hurdles now as a lack of confidence to put all of his pressure on the knee when making cuts and a hamstring that needs to be strengthened.

Because of the question marks, Nelson likely won’t know what teams are interested for a few more weeks.

“There will be a demand for proven receivers on the first few days of free agency,” he said. “It probably will be a lengthy process for me.”

Offseason analysts have mentioned him as a quality fit with the Cincinnati Bengals, Dallas Cowboys, Denver Broncos and New England Patriots.

The Bills could bring him back at a salary well below the $1.323 million tender. The 2013 minimum salary for a player with three accrued seasons is $358,000. That’s a wide gulf to negotiate savings.

Nelson said he has been spending much of his time researching the other 31 teams in the league.

“I want to make sure that whenever a team calls and says, ‘We’re interested in you,’ that I know the style of offense and how I’ll fit,” Nelson said. “If only three teams call, I want to know what kind of offense they run, how I can fit in that offense, what the overall scheme of the coaching staff is, what the quarterback situation is like, who the guys in the locker room are.”

Nelson is settling in for a long wait, illustrated by the decision to go to India next week with his two brothers to scout orphanages to help for a fledgling non-profit ministry.

“It wouldn’t be bad to come back and have a couple of job offers,” Nelson said.