"I think he realized it was a little different maybe, going into a start instead of coming out of the bullpen," Servais said. "Sometimes you get too much time to think about it, and I think he got away from pitching his game and what he does well, which is move the fastball around.
"He didn't have great fastball command, and he needs to have good command with his stuff. So we'll give him another shot."
He'll face Texas lefty Martin Perez in the middle game of the series. De Jong was acquired in the spring and was 2-0 with a 1.74 ERA in two starts for Triple-A Tacoma after being the Texas League Pitcher of the Year for the Dodgers last season, when he went 14-5 with a 2.86 ERA in 25 starts for Double-A Tulsa.
Cishek still working his way back
Reliever Steve Cishek is on a Minor League rehab stint with Tacoma, but the veteran right-hander threw a bullpen session at Safeco Field on Tuesday afternoon under the watchful eye of Mariners pitching coach Mel Stottlemyre Jr., as he tries to find his form following a long recovery from hip surgery.
The 30-year-old has a 4.50 ERA in five rehab outings with Tacoma and Double-A Arkansas, but he hasn't pitched since getting two outs on Thursday for Arkansas in the second game of back-to-back appearances. Servais said Cishek is healthy but he's taking a brief hiatus to iron out his delivery.
"It's just a mechanical thing," said Servais. "Mel noticed a couple things in his delivery that clicked right away when he made Steve aware of it, and the ball was coming out better. They may get together again and do another bullpen before he goes out and throws on another rehab assignment."
There had been hope that Cishek would rejoin the team on its last road trip, but the club will wait until he's right to add him to the bullpen mix.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.