A lot of physiotherapists will try to push you until it hurts, unfortunately. You mustn't allow them to. It will cause you unnecessary pain, it won't improve your progress and it may even slow down your recovery. Your job is to speak up for your knee and for yourself.
I know that you said you find it hard to speak up, but in this case you must. Remember that these people work for you, not the other way round. It is your body and your knee and they are not allowed to do anything to you without your consent. They're not the boss - you are! Tell them up front, right from the start, that you will not put up with them doing anything to cause you more than discomfort. Lay down your rules, right from the start. If you can, have someone there with you, to back you up and for moral support.
Best wishes!