He needs to explain how it "undermines" him doing his job, and how it "undermines" the people working under his supervision doing their jobs. Obviously comments about the Roger Stone case from other people on Twitter, etc., or from the liberal media don't "undermine" him or the total DOJ...just the President's comments.
That's easy. I makes the prosecution of any of Trumps perceived enemies look illegitimate and the result of Trump's criticism, not the rule of law.
Regardless of the facts.
It adds a new hurdle for the department and the prosecutors.