The 31-year-old had an 0.67 ERA in 42 games last season, including a franchise-record run of 29 consecutive scoreless appearances.
Sadler dealt with inflammation in the shoulder last year and missed nearly three months. He returned in July and emerged as one of the American League's top relievers. He frequently relieved Seattle's starting pitchers and wasn't fazed pitching with runners on base.
Entering his seventh big league season, Sadler is 6-4 with a 2.86 ERA in 101 games. His only save came in 2019 with the Los Angeles Dodgers.
Servais said he is keeping his options open in search of a reliever who will take over Sadler's role in the fifth and sixth innings.
"More games are won or lost in those innings than the late innings, so we'll look at different things as far as pocket guys," he said. "We might use a high-leverage guy in those spots than maybe we have in the past, but I don't have the plug-and-play.''
Seattle will use closer by committee, Servais said, unless general manager Jerry Dipoto swings a deal for an established closer.