After allowing just one run in 11 1/3 innings in June, the right-hander has taken a beating this month, surrendering 10 runs (eight earned) in six innings, including seven in his last two outings.
Handed a three-run lead on Thursday in Cincinnati, Shaw walked two batters, then served up a two-run double and eventually took the loss after recording just one out.
Just a day later, he allowed three runs on three hits in an inning of work against the Astros.
"The ball was flying straight, not a lot of action on it," manager Kirk Gibson said. "He's struggling a bit right now, he goes through that. We'll continue to talk with him and encourage him."