The most important reason for code review is not to fix the code but rather to transfer knowledge between developers. I've learned a lot of techniques for writing better code from suggestions from my code reviewers, and from reviewing other people's code.
Without code review, when will a junior developer ever learn anything from a senior developer?
It works both ways, too--it makes more developers aware of the others' work, what changes are being made and so forth. So it's not just expertise that gets transferred, but also the current state of the codebase.
In that case, it doesn't matter if you do the code review before or after they merge and deploy. You could read through the diffs in the commit log whenever it is convenient for you, and leave review comments for the other developer on their PR after the fact.
Without code review, when will a junior developer ever learn anything from a senior developer?