John Carlson hears the clock ticking on his career a formerly productive tight end defined recently by misfortune and injuries.

It has been three long years since Carlson was healthy and relevant during an NFL season. He is back with the Vikings after an unfulfilled 2012 season for the player and the team grateful for another chance -- perhaps his last -- to secure a permanent role.

"Every football player knows if you don't perform or are continually injured ultimately you're not going to be playing anymore" Carlson said this week. "You have to continually remind yourself in the grind you don't know how long it's going to last. Training camp's a grind but I love the fact that I can be out here and I'm thankful for the opportunity."

It remains uncertain how much playing time Carlson will get this season.

In training camp he has been active catching Matt Cassel's passes in the middle of the field where Carlson thrived early in his career with the Seattle Seahawks. He also has lined up at the H-back position a role Rhett Ellison and Jerome Felton mastered as lead blocker for Adrian Peterson last season.

The Vikings still are trying to determine what they have in Carlson who was projected to anchor versatile tight-end sets with Kyle Rudolph.

But the Litchfield Minn. native suffered a knee injury early in camp last year sustained his fourth concussion in October and was relegated to the background as Rudolph emerged as a dominant tight end and Pro Bowl player in just his second year.

"Certain injuries you can mitigate from training certain things happen that you have no control over like those random acts last year" Carlson said.

Carlson never could assimilate with quarterback Christian Ponder or offensive coordinator Bill Musgrave. Now he is working to make up for lost time and the Vikings are affording him opportunities to re-establish himself.

"No question -- it set John back a year ago" Vikings coach Leslie Frazier said. "But so far he hasn't missed anything. He's had good practices and we'll get him now to hopefully get to the ballgame on Friday night and get a chance to play with our starting unit some.

"We know he's a good player we know he's capable of making plays. We're going to try to give him as many opportunities as we can this preseason and try to get him involved. A lot of it will be based on what we get accomplished here at training camp and what we get accomplished during the preseason."