The loss of a history, the loss of his hopes that maybe they’d reconnect, the loss of someone who shared your pain even if you never truly talked about it. God forbid someone offer Optimus Sentinel’s former role as sub-commander or as Magnus because it’s going to feel like the cruelest joke the universe could ever play on him—“Here’s your dream, don’t mind that it was just pried from the cold hand of your buddy’s sparkless husk, after all, you wanted to be Magnus right?”
I could possibly see him holding it together a little bit but once it sets in that Sentinel is gone he is not going to be okay. He’s never been the type to show these kind of feelings to his team and they barely know the details of his relationship with Sentinel and how deep it truly ran. They don’t understand that while his was a glitchhead he was still someone Optimus yearned to reconnect with, even if he doubted it would truly happen.