Calvin Johnson retired in March after a stellar nine-year career. Just 30 years old, the former first-round pick admitted recently that the Lions' struggles played a part in his decision to leave the game.

"If we would have been a contender it would have been hard to let go," Johnson told E:60's Michael Smith.

The Lions were 7-9 last season -- which included an 0-5 start -- though the team won 11 games in 2014 and went to the playoffs. Since Johnson came into the league in 2007, the Lions have had just two winning campaigns, the other in 2011. In 2008, Johnson was part of the team that went 0-16.

"I don't know (how much the losing factored in), but what I can tell you, is that it's a big unknown because ... say you were winning, you win a Super Bowl and you know, you just never know how you'll feel," Johnson said. "I'm wasting my time trying to think about what could happen, what would have been. ...

"It's funny," he continued, "because there would be certain things. During the season I would be chilling with the boys in the cafeteria and watching something like that, and they talking about Kobe (Bryant). They were talking about how he was retiring, and I'm like '(expletive) I'm retiring too after this season' ... But nobody pay no mind. They thought I was BS-ing."

Johnson said that injuries were one of the main reasons he retired, noting that he suffered his "fair share" of concussions even though he was never listed as having one on the team's injury report.

"Concussions happen," he told E:60. "If not on every play, then they happen like every other, every third play, you know. With all the helmet contact, guys hitting the ground, heads hitting ground. It's simply when your brain touches your skull from the movement or the inertia, man. It's simple to get a concussion, you know. I don't know how many I've had over my career, you know, but I've definitely had my fair share."

A transcendent talent, Johnson was easily one of the best decisions former Lions general manager Matt Millen ever made. Johnson had at least 1,100 receiving yards seven times, and finished just 36 yards shy of 2,000 receiving yards in 2012. For his career, he had 731 receptions for 11,619 yards and 83 touchdowns. According to Football Outsiders' WR efficiency metric, Johnson finished in the top-10 among all pass catchers on six occasions, and ranked first in 2011 and 2012.