Pivetta and Velasquez both had FIPs of 3.80 or better last year. That is a track record that suggests they can be decent 4th starters at worst. They could be more consistent, but you could say that of everyone this year Nola included.
They should only be replaced if we have someone better. That is not Irvin. Eickhoff might be as good or better if he is healthy (again as good as a 4th starter), so he is getting a chance. I just see no reason to give Irvin an extended chance. I'd be more comfortable if they wanted to make him a 2-3 inning long man behind all of our right-handed starters.
As I said, I am perfectly comfortable trading for an upgrade if Pivetta and VV do not pitch well. They just deserve to pitch ahead of Irvin for now.