Trump found out about the special counsel after Rosenstein decided to appoint one, and then had a meeting with advisors to decide on how to respond. And then he spent the next day bitching on Twitter about it. Oh, and White House lawyers are looking into whether or not they can invoke a conflict of interest rule to weaken Mueller.
Doesn't sound like a whole lot of agreement to me. He 'agreed to it' the same way that people agree to things they can't control.