"He needs to pitch better," Angels manager Mike Scioscia said. "And he's struggled with that."
In the series opener against the division-rival A's, the 27-year-old left-hander was spotted a 3-0 lead after 2 1/2 innings and then couldn't escape the bottom of the third. He gave up a leadoff double to Josh Reddick, issued back-to-back two-out walks to Mark Canha and Brett Lawrie, served up a two-run double to Danny Valencia, walked Josh Phegley and gave up a bases-clearing double to Jake Smolinski.
Then, he exited.
Santiago hasn't pitched five full innings in either of his last three starts, a span that has seen him walk 10 batters in 10 2/3 innings. After making his first All-Star team with a 2.33 ERA, Santiago has a second-half ERA of 5.93.
"It's one of those times in the season when things go wrong and things kind of pile up on top of each other," Santiago said. "You try to make adjustments, and you overthink some situations and overwork throughout your bullpen days, and try to change things that you don't need to change. It's like a hitter. They go 0-for-25, and then they go 25-for-50. It's just one of those rough times, and I kind of let it pile up on top of each other, tried to fix some things and change some things I don't need to. Now I feel like I'm fighting myself mechanically."
This is the first time Santiago has been in a Major League rotation all season, and naturally his workload has gone up. He has already thrown 152 1/3 innings, 3 1/3 more than the previous career high he set in 2013, with all of September remaining.
Scioscia pointed to Santiago's unaffected velocity in saying he's "got enough left in his tank."
Through the first half, though, Santiago got by with an unsustainable strand rate of 88.9 percent, second only to Dodgers co-ace Zack Greinke and well above the Major League average of 73.1. Now, though, a lot of those runners are scoring.
Like the rest of his Angels team, everything is going wrong.
"It's weird," Santiago said. "But I feel fine. My arm feels great. All around, I think it's just a bump in the road."
Alden Gonzalez is a reporter for MLB.com. Follow him on Twitter @Alden_Gonzalez and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.