Overton made the Mariners' 25-man Opening Day roster, but he flew to Eunice, Texas, over the weekend to be with his wife in anticipation of the inducement of their first child's birth Tuesday.
Major League Baseball's paternity list allows players to miss one to three days. Overton said last week he expects to rejoin the Mariners on Thursday for their series finale with the Astros.
De Jong traveled with the Mariners to Houston and was available for Monday's season opener at Minute Maid Park. He has never been above the Double-A level outside of one Triple-A game in the Dodgers organization last year.
"This is the end goal and where you want to stay," De Jong said, looking around the visitor's clubhouse at Minute Maid Park. "It's going to motivate me every single day no matter where I am, to be here and stay here."
De Jong is considered a starter by the Mariners and will rejoin Tacoma's rotation after Overton returns, but manager Scott Servais said the club was looking for someone who could provide long relief if needed in the first few games of the season.
"We really like him," Servais said. "He's got work to do, no doubt. If we get him in these games, great for him, good experience, and we believe he'll handle himself well."
The 6-foot-4, 205-pound De Jong was acquired from the Dodgers on March 1 for Minor League shortstop Drew Jackson and pitcher Aneurys Zabala. He pitched in three games (two starts) this spring for Seattle, allowing five earned runs in nine innings, before being optioned to Triple-A on March 17.
De Jong was 14-5 with a 2.86 ERA in 25 starts for Double-A Tulsa last season and 1-0 with a 1.69 ERA in one start for Triple-A Oklahoma City. He was named Texas League Pitcher of the Year with Tulsa.
Overton, 25, was a surprise selection to the Mariners' 25-man roster as a reliever after pitching well in a starting role during Cactus League action, when he posted a 1-1 record with a 2.75 ERA in 19 2/3 innings.
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.