Kreilach's goal helped Salt Lake climb to fifth in the Western Conference standings, with sixth-place Portland hosting eighth-place LAFC on Sunday.
And it also exacted a small measure of revenge against Seattle assistant Freddy Juarez, who left as RSL manager several weeks ago to work on Sounders manager Brian Schmetzer's staff.
Salt Lake has won three of its five matches since Juarez's departure, and completed back-to-back wins for the third time this season after a 4-3 victory at San Jose on Wednesday night.
Seattle lost for only the second time in 11 league games while missing a chance to move above Sporting Kansas City and retake the Western Conference lead.
Stefan Frei made four saves to keep the Sounders in it on a night they conceded just their ninth goal on the road this season and 20th overall. Both marks remain the best in the league.
Kreilach put RSL in front three minutes after halftime following Aaron Herrera's excellent work on the right flank.
With Brad Smith defending, Herrera cut back not once but twice to create space for an outswinging service. Kreilach met it with a firm header at the back post that left Frei little chance.
Frei made an excellent sprawling save with his left leg to deny Albert Rusnak's first-time effort from a low cross in the 57th minute. A few minutes after that, Rubio Rubin juked his way past a Seattle defender on the right but fired just wide of the far left post.