With all due respects to the "humble banana" and your friend ,eating bananas is no way going to cure your knee pain
. But on the other hand you being a diabetic and its glycemic index
being high its definitely going to rise your blood sugars. I would request you to meet your diabetologist
and clear the doubts regarding this. As for the knee pain by the history that you have given it seems like a more strain related issue. Considering the fact that you haven't had a fall while playing tennis simple ice compressions, rest and analgesics should be enough . I suggest discontinuing your tennis for 10 days.