Injury, not desire, decided Justin Smith's fate

Justin Smith becoming the third member of the San Francisco 49ers defense to retire this offseason was not a shock. It was expected.

Yet it is still a hit for a 49ers defense that lost not only inside linebackers Patrick Willis and Chris Borland to retirement, but also cornerbacks Chris Culliver and Perrish Cox and outside linbebacker Dan Skuta to free agency. Defensive end Ray McDonald was cut with two games to play last season and signed as a free agent with the Chicago Bears.

As noted by ESPN NFL Insider Mike Sando, the front seven players in their 3-4 defense with the most defensive snaps since 2010 are Smith, Willis, McDonald and Ahmad Brooks. Plus, only three defensive starters remain from the Niners' Super Bowl team of the 2012 season: Brooks, NaVorro Bowman -- who is attempting to come back after missing last year with a left knee injury -- and Aldon Smith.

So even if the Niners have been preparing for Smith's last ride for a few years now -- they have drafted 10 defensive linemen since 2011, including Oregon's Arik Armstead in the first round this year -- Smith leaving is a big hit because in the end, not even the presence and elevation of his longtime position coach Jim Tomsula, who was promoted to head coach in January, was enough to sway Smith to return for a 15th NFL season.

Not when Smith’s body simply gave out.

Smith, who played on the right side of the line, said in a Monday conference call that a left shoulder injury suffered in a 2013 training camp drill with then-left guard Mike Iupati never truly got better.

"A piece of bone detached," Smith said. "I just knew it hurt like hell."

With his position on the line, most of the contact he endured was on that left shoulder.

"It wasn’t responding," Smith said. "If you don’t have the tools, you can’t do the job. It’s time to go.

"There's arthritis build up, wear and tear, yada, yada, yada ... it’s a young man’s game."

The 49ers gave Smith, who turns 36 on Sept. 30, as much time as he needed to come to his conclusion. That he made it official the day before the team begins OTAs allows the Niners the opportunity to address the huge hole that exists now on the defensive line, one that still exists even though they signed Darnell Dockett, who turns 34 on May 27, in the first wave of free agency and drafted Armstead.

"That guy has taught me a lot as a coach by seeing the game through a player’s eyes," Tomsula said of Smith. "Not just a player's eyes, but that player's eyes.

"He doesn’t compare himself to anyone; he compared himself to himself, the bar he set. And if he didn’t feel like, physically, he could play at that level, he wasn’t going to do it."

Smith leaves with one year remaining on his contract but admitted when he signed his two-year extension in 2013, he did not think he’d finish it out. He gave himself a "20 percent" chance of playing 15 years in the NFL. The five-time Pro Bowler played 14, only missing three games.

"When you're on bald tires, you're on bald tires," he said. "It's a young man’s game. It was just time for me to move on.

"It was a great ride."

And with that, the Cowboy rode off.