I'm curious, why does this strike you as hard to believe? I take it you forgot about the fact CE-P0 had lost his arm...
Leia...She's going to fall down the stairs and shatter her arm and then C3PO will operate on her, giving her one of his arms. It will be fuel for the transhumanism agenda.
Phasma getting her hands lopped off in that matter would be on par with the cheeseball de-handing of Count Dooku.
Luke gets his other arm severed by Kylo Ren, then Kylo Ren dies saving Rey. Then Leia turns to the dark side after Han and Ben die in two movies.
Rey severed the Rathar's tentacles in Force Awakens. Maybe she'll chop off space horse's legs in this one.
That's only because all of the lightsabers were apparently set to 'stun' in Episode VII for some indiscernible reason.
Some middle ground, just getting touched, a la Kenobi on the leg should not put you out of commission. Seriously, though Finn, should have clearly died from that blow and Kylo was lucky his face is one piece.
I noticed that in episodes V and II, a good guy and a bad guy both lose an arm/hand. Episode II: Anakin and Zam Episode V: Luke and the Wampa At this point, Rey and Phasma losing an arm/hand in VIII doesn't seem too far fetched to me.