Adam Ruzek (Patrick John Flueger) has become an indispensable part of Chicago PD. The character drove most of season 10, whether it be through his relationship with Kim Burgess (Marina Squerciati) or the undercover mission that resulted in him being shot in the stomach.
It's hard to imagine a version of the show without him in season 11, but that's exactly what we were given during most of season 4. It's easy to forget about, since the character still appeared, but there was a long stretch in which Ruzek's future with the IU was unclear.
Ruzek lost his position in IU during Chicago PD season 4
Ruzek went on an undercover assignment during season 4, but when he came back to IU, he found that his position had been given to another officer, Kenny Rixton (Nick Wechsler). Ruzek asked Hank Voight (Jason Beghe) to help him out, but there was nothing that could be done, and Ruzek had to return to uniform.
It was an unexpected development for the character, and one that was made all the more serious by the fact that it took several weeks to resolve. He missed four consecutive episodes, which suggested that he may never come back to the IU.
PD showrunner Matt Olmstead told The Hollywood Reporter that it was important for the audience to feel Ruzek's absence, and legitimately be concerned for his future within the police department:
""When we decided to have Ruzek surprisingly leave Intelligence, we didn’t want to have him come back the very next episode and have it be a complete wank storyline for the audience. So we really wanted to follow through on the threat and we were also born from the reality of the situation.""
Ruzek was briefly replaced by PD officer Kenny Rixton
Olmstead also pointed to the fact that Nick Wechsler was giving a stellar performance at the time, and he wanted to give the audience the impression that his character was a serious threat to replace Ruzek in the IU workplace.
"[Nick] did a great job for us," he recalled. "And it was almost timed to where the audience is thinking, 'Oh, Ruzek’s not back the next episode, he’s not back the second episode — he might be gone, gone.'"
Thankfully, Rixton wound up taking a position with another division, and Ruzek was free to take back his role within the IU. It hasn't been smooth sailing ever since (Ruzek has definitely questioned the job at various points over the years), but he wound up being one of the most valuable members on the team.