因为 here is not good.我妈因为去做了膝盖手术，所以他现在不能走路。（书面，正式
Thanks T.D! Yes, it is that she has trouble walking and not that she cannot walk at all.没法走路；不能走路 = cannot walk at all.
have trouble walking =走路不方便(one way to casually say it); 行走不便(formal)
Actually, as I was writing it, I was thinking that was perhaps inexact. She is having knee problems because of her +"having had" a knee operation, and is in a recovery phase. Her condition is temporarily worse than what it was before the operation.because of her knee operation = I understand it as she's got a problem with her knee and took an operation to cure it. So it is not the operation that prevents your mom from walking, but the recovering process. @#2