Unfortunately, in our Skype English classes online students have issues translating the verb ‘operar’. We don’t simply translate the word ‘operar’ just by the word ‘operate’ when we’re talking about getting some part of your body fixed. We have to add the preposition ‘on’. Thus the verb in English is ‘to operate on’. So we wouldn’t say ‘I will have my knee operated tomorrow’ but we would use the following phrase: ‘I will have my knee operated on tomorrow’.
In addition, and perhaps more simply, we have the option of not using the phrasal verb ‘to operate on’ and instead we can use ‘to have’. Thus: ‘I will have a knee operation tomorrow’.