Ok, WHO HAS HAD KNEE TENDINITIS?
If you haven't had it, do you KNOW about it? This is a condition that, in its current state re: Portis, causes pain, discomfort, some possible minor inflamation. But this is something you CAN work through... it "acts up" somewhat like a tight back, but in an area where it limits you far less than say an irratated back would.
As stated, this is minor at this point. Furthermore, tendinitis is not something that would likely sideline Portis from playing in a game though it would reduce his practice time.
Basically, the guy has tendnitis. The only way to fix that ill is to not run. For like, well, the year. So he is going to play, and he WILL irritate this injury and you WILL hear about it - but he will play. This is nothing like what Westbrook has or others where the injury threatens playing time significantly.
ETA - I did have knee tendinits toward the end of high school and in my first year of college. It hurt, but I could definately play through it. Mind you, I was NOT on an NFL regimen (DIII Football). Now, it was one of the deciding factors in me hanging it up, because I can tell you the constant discomfort/pain was bothersome... but I was a DIII player wanting to take bong hits, not an NFL stud looking to remind people of how good a player he is in the NFL.