Honestly, it's going to be extremely difficult to regain your prior level of fitness. Taking off 6 kilos, in and of itself, is probably going to take you at least 3 months if not half a year, and you've got the additional constraint of also improving your physical strength and stamina. Add to that that you're not as young as you used to be, and it adds up to that a more realistic goal is to be the best you can be at your current age rather than to chase that past dream.
Right, with that out of the way, the first thing I'd note is that you will likely want to see a doctor, maybe a physical therapist, about the knee. It sounds like there may be some lingering trauma there from your prior years of playing that's been exacerbated by your increased weight and decreased conditioning. Knees are always tricky things regarding injury, and I'd recommend the doctor both to prevent more permanent injury and also because even a minor recoverable injury can hurt your motivation to get fit and stay fit. Complicating things, as a former athlete, you're used to being in good shape and will have to actively check your expectations in workouts to prevent injuring yourself and burning yourself out.
I feel like this is sounding overly pessimistic. That is not my intent. As someone who has been in shape before, you're also in a good position to get back in good shape, if not at the top of your game as before. Specifically exercising for improving your basketball skills, you're going to be looking at cardio, of course, for endurance and for overall caloric burn (cardio isn't as efficient for weight loss as High-Intensity Interval Training or lifting weights, whether bodyweight or external, both of which will boost your metabolic rate more, but it's a lot easier to maintain for a longer amount of time). Plyometrics is pretty much a given since you're looking for explosive power in your limbs, particularly jumping skills. With both, you're going to want to start slowly. Normally, the advice would be to listen to your body, but you're probably used to pushing your limits, so at least for the first week or two, you're going to want to follow a relatively easy plan, probably half of what you automatically assume you can do. If you can do that for a few weeks and you're not waking up in pain, you can start scaling up and listening for what your body is telling you.
You will also want to address diet. At first, you're not going to be burning that many more calories than you did when sedentary, so you're not going to be sharply increasing your caloric intake, but you're going to want to keep track of what you're eating and how much. Reduce junk food intake as much as possible and ensure that you're getting sufficient amounts of protein in your diet. When your workouts get more intense, you're going to want to increase your intake a bit to accommodate the extra energy needed to build muscle, but continue tracking things to ensure that you're not eating 500 calories to compensate for a 300 calorie workout.
Best of luck! Like I said, you've got some unique advantages and pitfalls ahead of you as someone who previously professionally played sports.
