He wasn’t awful the first start.
Also don’t exactly see guys like Nimmo, Lindor, Alonso in triple-A.
Manning’s stuff+ actually looks decent, all three pitches graded average or better in 2023. Most notably the slider was well above average. The 7.9% SwStr% seems to be the issue.
He gave up way too many fly balls and hard contact, bottom 9% percentile in giving up barrels isn’t a good recipe. Fly balls + hard contact + low swinging strikes.
I don’t watch him enough to pinpoint the issues, but possibly needs better command as pitches getting too much plate, or there’s not enough separation between his pitches to induce swinging strikes.
You can kinda get away with it in triple-A but not the big leagues.