When the Astros signed Rafael Montero to a three year, $34.5 million deal after the 2022 season, he was coming off a fantastic season where he posted a 2.37 ERA in 71 appearances. He missed a bunch of bats, had a reasonable walk rate, and there weren't any pitching peripherals that indicated that what he was doing was smoke and mirrors. If he was truly THAT guy, $11.5 million a year isn't completely unreasonable for a bullpen arm of that quality when looking at the market.
However, 2023 was a disaster for Montero. In 68 appearances with the Astros this past season, Montero put up a 5.08 ERA with a 4.46 FIP and getting to that point required him to have a good second half. Some of that was bad luck on balls in play as evidenced by his underperformance of his FIP, but he still wasn't anywhere close to the 2022 version of himself.
In hindsight, this deal was always a bit questionable given Montero's track record. He had never appeared in more than 44 games in a season before 2022 and before that magical season, he averaged a 5.18 ERA during his time with the Mets, Rangers, and Mariners. Rafael also has always been a perennial underachiever when it comes to his FIP which could be indicative of a more fundamental problem beyond "bad luck".
This is a cautionary tale that buying into one good season for a reliever that is already north of 30 years old isn't a great idea. Now, Houston is stuck paying him $11.5 million for the next two seasons and that money would have gone a long way to beefing up the rest of the roster.