Maybe I am too biased about Zaniolo and look at the situation with rose-tinted glasses, but to me he is not that injury prone. It sounds crazy saying that after 2 ACLs, but how many guys continue to have them? Two is about as max as you can go, unless you are Giuseppe Rossi.
First ACL was a result of a heavy blow and collision, that can happen to terminators like CR or Zlatan. Second I do not know the nature of.
Aside from that he does not have the nagging muscle injuries that plague us the most.
My point is ACL is a serious injury, but it is not something that keeps coming back. And if his knees are permanently shot then that would come up during the medicals and you do not sign him.