Having a knee injury as a triathlete/endurance athlete is a problem. A common cause I see as a personal trainer and triathlete is over used quadricep muscles(quads) and weak Gluteus muscles(Glutes). What I recommend is stretching out the quads accompanied with hip and glute work. Such as one legged squats, lunges and more specific exercises that isolate the glutes and the Vastus medialus (VM). On legged exercises will target these muscles. I find that adding a more dynamic element like a step up on to a high bench is necessary in order to build functional strength.
Exercises to strengthen the Glutes and create stabilisation.
step ups (one leg at the time)
Lunges and squats with Resistance band.
Adding these exercises into a strength programme is a good way to reduce the risk of injuring the knee.
Having said that!
Depending on the state of the injury, these exercises may have to be adjusted depending on the person and where in the recovery process they are.
If you are an injury free person or feeling small pain in the knee and want to add these exersices, I would sugest to have a professional look at your technique whilst performing these exercises. Done the wrong way might worsen the injury.
Hope this helps