Technically summer is officially June 20 until September 22, so Barnes has only been lousy in part of this summer -- the part that matters most so far, namely when the first-place Red Sox blew the division lead.
iortiz astutely cites, "command and control" as the issue, and from my observation, Barnes has lost his curve when it matters. More are bouncing, less are snapping in the zone, and batters have been sitting on his fastball. And I'm not talking about Semien hacking first pitch, but Springer swinging from the heels on 3-2 because he knows dead red is coming.
In reverse, this is why Eovaldi continues to improve. In the past it seemed a mystery how a guy who throws 100 mph could get hit so much, but Eck and co. now rave about the mix of Nate's "five-pitch repertoire".