If in doubt, use an upper body ergonometer. If that is not available, I recommend to people to use the ‘old’ Schwinn bicycles with the arm levers.
If otherwise cleared for exercise, starting with aquatic exercise is a good option.
You use a rather generic term ‘knee injury’. Do you have any PT advice? That’s where to start. If you are looking for land-based exercises, it will have to be in dialogue with the client and what works for them. It is always most important to start on new modalities for a very short period of time and judge about it the next day even if nothing has been bothering during the exercise itself. Then take it from there with appropriate progression.