"It was a pretty clean image apparently. Nothing to be overly concerned about, but we're just gonna back off on him," Mackanin said.
Eickhoff, 27, was 4-8 with a 4.71 ERA in 24 starts this year. His average fastball velocity had dropped from 91.5 mph in 2015-16 to 90.4 mph this season. It averaged 89.4 mph in six August starts.
The Phillies are hopeful Eickhoff can return to his 2016 form, when he went 11-14 with a 3.65 ERA.
The club thinks he can benefit from some time off, as Aaron Nola did when he sat out the final two months of 2016 with a strained right elbow.
The 24-year-old Nola has become the club's ace, going 10-9 with a 3.46 ERA in 22 starts this season after posting a 4.78 ERA in 20 starts in '16.
"[Nola] came back in a blaze of glory and has been a different guy since," Mackanin said. "So sometimes that's what happens. You've got to back off on the guy, especially a younger guy."
Mackanin said Eickhoff is a good bet to start in next year's rotation.
"I'd like to think he's going to be in the rotation, because he was so solid for us last year," Mackanin said. "He's definitely a top candidate for the rotation."
Jake Thompson will be recalled from Triple-A to take Eickhoff's spot in the rotation, and will start Sunday against the Marlins. The 23-year-old right-hander is 1-1 with a 5.40 ERA in six games (three starts) with the Phillies this year.
Patrick Pinak is a reporter for MLB.com based in Miami. This story was not subject to the approval of Major League Baseball or its clubs.