She protects him and guides him and inspires him, so Michonne losing the younger Grimes male would be very much like once again losing one of her biological children.
And we’ve seen how those past tragedies have continued to haunt Michonne at times, though she’d been as stable as ever in the first half of Season 8.
In the past few seasons, going back to even before Michonne started up her courtship with Rick, her relationship with Carl has grown and expanded, and she has been a solid stand-in for Lori, serving as another mother figure for Carl to look up to.