We'll see from either qualifying or race exactly what the failure for Button was, they are saying an ers failure on twitter, but a failure to mgu-k/h can kill the engine as well. They said they were replacing the engine which makes you think ICE but really could be any part. It's a shame everyone uses engine interchangeably, would be nice if everyone in the sport stuck to the same more precise terminology. Alonso before the weekend said there was definitely no engine change, but he was talking specifically about the ICE as they intended to keep that(but failed to) but the turbo/mgu-h was definitely being changed due to the updates.
Either way I hope it wasn't the MGU-H that went after only 3 sessions if they spent tokens attempting to fix what was the most problematic part on the car all season. New reliability improved part with tokens spent breaking almost immediately would be a disaster.