I may be wrong, but I don't think it's necessarily on the coach. Coincidental things happen often.
dempster's an older pitcher who's been worked really hard. It's hardly unusual for a pitcher to come up with arm trouble, and be the kind of ineffective that Dempster has been when trying to throw with a bad arm. Rothschild wasn't using an 86-mph Dempster while he was here.
Garza has been racking up innings for years, and his effectiveness had already dropped last year. His ERA+ was 101, well down. He's perhaps at a point in his career where he's not getting better but his arm might perhaps be getting worse. Is it astonishing that a guy who had deteriorated last year and was basically an average pitcher overall would have a month where he's not been very good? Happens to average pitchers all the time, even when healthy. And perhaps after the last few heavy years his arm isn't quite what it was 2-3-4 years ago.
Zambrano has put a ton of wear on his arm, and had been in a gradual decline for years until his 2nd-hand anti-HR rebound last year. But that anti-HR spell last summer was quite out of line with how he'd pitched for Rothschild earlier in the year and during the many years previous. It's hardly surprising that he's not gone all month without a HR.
Rothschild NEVER had a guy like Russell starting. I don't think it's on the coach when Russell is bad.
Next week maybe they'll call up Jay Jackson, who was highly hittable and was a HR-factory last year in the minors. I don't think it will be on the coach if Jackson comes up and gives up HR's and looks overmatched.
I thought Rothschild was an excellent pitching coach, by the way. I thought he was very good. But I'm not sure the current debacle is Riggins.