Brewster's BAU agent departed the show in last night's (May 16) season finale, with Messer telling Zap2it that "there are talks" ongoing about a replacement character.
"It's unlike most shows in that sense that changing out characters can be a really difficult thing to do, and we've learned that," she said.
"I can [also] see this team going on as they are, because one of the hardest things for all of us writing it and fans is to introduce a new person to that round table. It's such a family."
Messer added that it had been "tricky" when new character Ashley Seaver (Rachel Nichols) briefly replaced JJ (AJ Cook) in the show's sixth season.
"She was so young," explained the showrunner. "It's OK for... Reid (Matthew Gray Gubler) because he's super-special genius boy, but it's hard to make excuses for that [age gap] when we've said for so long that it's such an elite group."
Criminal Minds returns to Wednesdays at 9/8c in the fall.