Here's one out of left field. Hisashi Iwakuma has been placed on the 15-day DL with a right lat strain, retroactive to 4/21. There's no telling when he injured it, or how long it's been bothering him. If it dates back a bit, there's your aha moment for his struggles.
No word on who's coming up to replace Iwakuma, but it seems fairly obvious it will be Roenis Elias. In the mean time, Lucas Luetge will come up to help in the bullpen until they need Iwakuma's turn, which is scheduled to be Sunday afternoon. Taijuan Walker could go on regular rest on Sunday, thanks to their day off on Thursday, so an immediate replacement isn't required.
Lat strains are a tricky business, and the recovery time depends on the severity and ability to avoid setbacks. Last year, Doug Fister suffered one at the end of March and didn't return until May 9th. James Paxton strained his lat during last year's home opener, but he suffered some setbacks along the way that kept him shelved all the way until August. We're likely looking at somewhere between five to eight weeks for this, assuming it's of a relatively normal severity, and if so, history would seem to indicate that it'll be on the longer side of that window.
It's terrible news if you operate under the assumption that Iwakuma was going to return to his former self, but after last year's awful finish and a shaky return out of the gate -- of which the struggles were not clear, it might not be the worst thing. An unhealthy and unproductive Iwakuma might actually be worse than Roenis Elias. This is why the Mariners held onto J.A. Happ instead of Elias -- and now they'll see that safety net used.