I'm not saying we're short of players, but we're short of quality outside of the first eleven required to get automatic promotion.
In defence we have McShane who is always capable of a blunder and Cooper who is a good player, but is he good enough for a team that want to challenge for promotion to the PL? Perhaps not.
In midfield we have Cairney who blows hot and cold and has never been quite comfortable at this level (certainly not as good as when we were prem), Brady who's probably off in Januray anyway but again he's as likey to have a bad game as he is a great one. Pusic looks the part and i've rarely seen him play so i won't comment on. Oli hasn't played for us in **** knows how long so we don't know what he'll be like. Garcia's just come back from another long term injury but yes, he's great at this level assuming his injury hasn't affected him much.
Up front we literally have Adebola as backup, without slating him considering he's not really put a foot wrong since signing, it isn't exactly ideal.
And we all know the keeper situation.
So i agree that we have a squad that's about the right size, but 2 or 3 injuries to the first team players and i think we'd notice a dip in quality. Which in answering the question of the OP, won't get us automatic promotion. I still think play-offs are well within our grasp without big investment, even with a few injuries.