The 31-year-old is working back from bursitis in his right shoulder and threw a strong bullpen session Tuesday in Coors Field in Denver.
"Felix was very good," Servais said. "Better than I was expecting. He threw all of his pitches, not just fastball and changeup, and throwing a bullpen in Colorado can be challenging, just to get the ball to move and do what he does. [Pitching coach Mel Stottlemyre Jr.] said it looked great."
Haniger was off to a red-hot start in April before straining the oblique in his right side. His attempt to return two weeks ago was stalled when the issue cropped up again during workouts, but the 26-year-old is getting back to that point again and did some running and throwing on the field Wednesday, and will take batting practice with the team on Friday, Saturday and Sunday
"If that gets revved up, he could possibly go out on a rehab on Monday," Servais said.
Haniger won't need to wait four days between Minor League outings like Hernandez, so he conceivably could get sufficient work in to rejoin the Mariners in mid-June during their next road trip.
"It's probably frustrating for him," Servais said. "He was really amped up 10, 12 days ago and when he had a little setback there, it does get a little frustrating. But he continues to work his tail off. He's the first guy in here every day. He's very structured and I think understanding more is not always better, he's tried to work a little smarter these last five, six days."
• Hisashi Iwakuma also threw a bullpen session on Tuesday in Colorado, but the veteran right-hander remains a little behind Hernandez in the recovery process as he's not yet thrown all his pitches while he continues recovering from a shoulder issue. Iwakuma played catch pregame on Wednesday, but is not scheduled for a simulated game yet.
• After being taken out of Tuesday's game in the second inning with tightness in his right calf, designated hitter Nelson Cruz was back in the lineup Wednesday. The veteran started in right field both games in Colorado, but won't need to play in the field again until Aug. 21-23 when Seattle faces its final Interleague road series in Atlanta.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB. This story was not subject to the approval of Major League Baseball or its clubs.