I think the versatile Song will show himself to be a very useful player, just as Mascherano did. Even with all the complaining about Mascherano/Song lately, there is no way you can argue that this team wouldn't be in huge trouble without them. Maybe you can make the argument somewhat with Alba as well. But Tello/Montoya/Bartra? They could be gone all season and it wouldn't have anywhere near the effect.
Now if we're talking longer term, of course the young players could very well prove themselves to be the best signings, but in terms of which player we need right now, I gotta go with Song along with Alba in a close second.