So it is a problem with at least one of his knees. 
How on earth could he possibly believe that he'll still be competing in 2016 given his current physical condition?

from what i heard about tendinitis,
it is a permanent case for sure but not a career threatening issue at all.......it flares up every single time he packs his schedule heavily and plays restlessly.......tendinitis interrupted rafa's career several times till now starting right from the age of 17.......
so he will have these breaks once in a while as long as he keeps messing with his schedule foolishly but that doesn't mean he is breaking down year by year........
no surprise that he is optimistic about 2016 because he can easily hold up till 2016.......tendinitis does not damage the bones, it is just an issue with the tissues connecting muscles to bones........so i don't see any breaking down as the tissues can be restored through treatment........
the common treatment for this is by using totally non-steroid drugs, blood replacement to heal the tendon, rest and gradual return to exercise........
the only thing is that he keeps pushing it too much and landing himself into trouble........