How much does a TDD aproach substitute a debugger?

It doesn't.
They have two different purposes.
The tests in TDD are there so you can find bugs in other parts of the code before publishing it, the debugger is there to help you solve the bug.

@FMashiro of course they have different purposes but they influence one another. I can think of bugs found during test phase, bugs which you can immediately solve without any debugger. As for other bugs you definitely need the debugger. I don't think a yes/no answer as yours is suitable.

@tomeb while you are right, your question was about knowing if it could be a substitute, and to that question the answer is no. No way around it.
You won't always need a debugger but it can't substitute TDD and TDD can't substitute a debugger.

@FMashiro "how much" meant an open question and an open answer. My bad if this was misunderstood. The opposite substitution is out of the question.

@tomeb my bad, let me answer it this way then:
"not at all"
That should fit your "how much" criteria.

Sorry to be so blunt but it really doesn't, not at all, not even the tiniest bit.

@FMashiro as you wish :)