It was announced on a live stream by Andy of Lets Talk FPL. He got a message from someone about 5mins before FPL lockout at 3:30am this morning and the FPL site went into meltdown as everyone traded Gabriel > Saliba.
He wasn't sure, but it didn't allude to injury. They have just signed Timber who plays CB (but covered LB in the Community Shield) so if it's true then maybe Arteta has other plans at LB. Gabriel has been a mainstay so maybe it's not.
It's one of those things that really only affected FPL, but not greatly as swapping Gabriel > Saliba could go either way anyway.