Monaghan had announced his exit in October 2018, and then we watched it play out over the course of season nine, which was also Emmy Rossum's final season. 

While Rossum appears to have actually exited, we soon learned that Monaghan would actually return for season 10. 

"It got to the point where I wanted to step away for creative reasons as well as business, which many other actors had done on the show years earlier," Monaghan told reporters at the TV Critics Association winter press tour in February. "But I took some time away, and I talked to John Wells, the showrunner about possible things we could do with the character in the future, and we got to a place both creatively and financially where I felt comfortable coming back, and much of that had to do with what we were going to do with the story."

At that point, that story hadn't been written, but Monaghan said he had hopes for his character beyond relationship troubles.