Seigler got his first start on July 2nd and appeared in 34 games (17 starts with a 13 W - 4 L team record) accumulating 73 PA of 50 wRC+ before being optioned to the minors.
Over that same time frame Monasterio has appeared in 38 games (20 starts with a 12 W - 8 L team record) accumulating 84 PA of 150 wRC+ and remaining with the team.
So Monasterio has gotten more consistent playing time, been better as an individual, but it didn't really result in the team winning more.
Seigler got slightly less PT, played way worse, yet the team performed better when he got his starts.
How can one guy be 50% below average as a hitter and the team do better when he starts than it does when the guy who is 50% above average starts? Because one spot at the bottom of the lineup is about the 11th to 13th most important thing in any game after the eight spots above it, the starting pitcher, the closer, and maybe a couple other pitchers along the way too.
Ortiz will have more chances in the field than Monasterio will have at the plate is another reason why Murphy continues to choose defense over offense at the SS spot.