Chicago Fire is prone to benching a character for an episode at a time. It's become a tradition over the past few seasons, with some vague lip service paid to why and when the character in question will be back. "Cut Me Open" was a different beast entirely, though. Instead of one character, the Chicago Fire episode was missing four!
We're as surprised as you are. Christopher Herrmann (David Eigenberg) was asked to man the store, so to speak, because Chief Dom Pascal (Dermot Mulroney) had decided to take a personal day. It's a bit odd to see an episode without Pascal, given his elevated position, but given how much he's been through in recent weeks, we get it. Then we learned Sam Carver (Jake Lockett), who has decided to transition to part-time shifts at 51, was also missing for the day.
Chicago Fire was missing both of its medics

Missing two vital characters is more than enough reason to raise an eyebrow, but before we could do so, Chicago Fire confirmed that our favorite medics, Lizzie Novak (Jocelyn Hudon) and Violet Mikami (Hanako Greensmith) were also missing in action! There are a couple reasons why such a staggering number of 51 employees were missing. One of them is narrative-based, and the other has to do with recent behind-the-scenes developments.
The narrative reason for being down Pascal, Carver, Novak and Violet is so 51 could look extra thin when the performance proficiency expert came by to survey it. The expert had been sent to make cuts on behalf of the Chicago Fire Department, and the lack of employees made a situation that would have been just stressful both stressful and compelling story wise.
Herrmann had to call upon the few firefighters who actually were on shift to help deal with the expert breathing down his neck, and they even managed to have some fun with the expert by forcing him to engage in physically taxing drills. It was a case of writers using a perceived weakness (lack of people) as a storytelling strength. It was also a case of fiction mirroring reality.
Firehouse 51 will soon be facing roster cuts

The behind-the-scenes reason for four characters being absent also had to do with budget cuts. NBC has renewed Chicago Fire another season, but the network has decided to whittle down the costs of episodes by benching main cast members 2-4 times a year. Or, in the cast of people like Jake Lockett and Daniel Kyri, writing them out of the show's future entirely.
Chicago Fire is feeling the squeeze of cutting costs both onscreen and off, and this came to a head during the end of "Cut Me Open." The expert confirmed that he will be making cuts to Firehouse 51, and even earmarked Herrmann and Mouch (Christian Stolte) as the most likely candidates based on their ages.
This disconcerting revelation is either a warning of what's to come, or a bait-and-switch for the two exits we already know are around the corner (the aforementioned Lockett and Kyri). Either way, we're going to be lamenting a permanently thinner 51 roster come season 14.