During an interview with The Hollywood Reporter — alongside series star William H. Macy — Shameless showrunner John Wells opened up about Rossum's departure and what he plans to do to address it in the show.
"It's painful, wonderful, exciting and a little frightening. We're rewriting the end of this season. I'm rethinking episodes 13 and 14, so we can deal with Fiona leaving in a way that feels true to the show and also sets us up for more storytelling in the future," he explained.
"Everything that we'll do with Fiona leaving and then hopefully returning at some point in the future — should Emmy choose to — will be based on that basic premise that these are damaged people who still really love each other and pull together to get through things," Wells continued.
Macy then chimed in, saying that while it is "sad" Rossum is leaving, "people have been moving in and out, and they tend to come back."
"I can't tell you how close these imaginary circumstances are to real life. It doesn't feel like goodbye forever," he added. "Fiona's got to come back. She's going to leave clothes there!"
Wells was also asked about how "prepared" he was for a major cast member to exit and he replied, "Not to suggest that I won't miss Emmy, but the reality is that everyone is ultimately replaceable. Fifteen years of ER proved that to me. We went through three separate casts."
"The original Shameless' Fiona left after six episodes," he explained, citing the show's BBC counterpart which came first. "The bigger surprise has been that we've gone on this long! We've done so many stories where we've felt like, 'Well, this will be the end of the series.'"
Even though Rossum's exit was a big surprise to fans, Wells revealed that he knew she had "been considering it for a while."
"I tried to encourage her to come back, because we want her here, but she has other things that she is excited about doing — the Angelyne show [based on a past Hollywood Reporter article] and a few things that she's directing. She came to the definitive decision a few weeks ago. We'll miss her," he said. "At the same time, there are a lot more Gallagher stories to tell.
Shameless airs Sundays at 8 p.m. ET on Showtime.