The move was made retroactive to Friday.
Right-hander Brian Schlitter, 33, was recalled from Triple-A Las Vegas in a corresponding move. The right-hander was 4-1 with 10 saves and a 3.89 ERA with Las Vegas.
Treinen, who was an All-Star last season when he saved a career-best 38 games, is 2-2 with 16 saves and a 4.08 ERA this season.
"It is what it is,'' Hendriks said Saturday. "I'm happy to be doing that. If it's in the ninth, it's in the ninth. If it's not, it's not. It doesn't change my mentality.''
Hendriks is 3-0 with a 1.49 ERA.
The Associated Press contributed to this report.