The Brewers haven’t faced many LH starters this season, but for his career Turang has been less effective against LHP than RHP. So, for one isolated AB, Monasterio arguably has a better chance of getting on base and starting a rally than Turang.
To start the game the analysis is significantly different. For one very important thing, Turang is an elite defender at an important position, something Monasterio is not. Having that elite defender might easily be more important than a relatively small difference in ability to get on base.
But, once the team finds itself 5 or 6 runs behind, the importance of defense falls away and getting base runners takes precedence
If you want you can quibble about pinch hitting for Turang at that stage of the game. But, IMHO, doing it when behind by 5 runs does not logically lead to the conclusion that Monasterio should start over Turang against left handed starters. I think the game situation significantly changes the analysis.