Over the many years that I have been writing sports, I’ve seen a lot of football players hampered because of knee injuries. Sometimes it is late in their career, and sometimes early. But usually when a guy isn’t playing well, it has something to do with his health.

Matt Kalil, the Vikings’ third-year left tackle, played last season despite a very bad right knee. He had surgery in the offseason to repair the knee, but it still bothered him at the start of the season. Kalil isn’t one to make excuses, but you can tell there are still some issues with the knee.

Medical advances over the past few years have been amazing, but that doesn’t mean that a player can immediately block and tackle like he does right away. Earlier this season Kalil said the knee pain during training camp caused him to pick up a few bad habits.

Vikings center John Sullivan has been one of the best centers in the NFL, and he is a good example of how it takes a while to rebound after surgery. He had a very ordinary season last year after having microfracture surgery on his left knee in January, 2013. This has been a much better year for Sullivan as he recovers from that serious surgery.

Kalil is being ripped from every side and if you watch him walk, he doesn’t walk like he used to walk, at least in my opinion. As a rookie in 2012, he was one of the top offensive tackles in the league and made the Pro Bowl. The knee bothered him all last year, and his play suffered.

I’m convinced it is giving him a lot of trouble this year, and that’s the major reason he’s not having a good season. But for some reason, other Vikings offensive linemen are having their off-days too. But the attention to them and when they fail to block or cause a quarterback to hurry doesn’t seem to get the publicity that Kalil does.

Now right tackle Phil Loadholt is out for the season, as is starting right guard Brandon Fusco. This has been a rough season for the Vikings’ offensive line.