I've thought about that .. how/who we are going to replace him/with. We've just lost 31 goals from Suarez (plus assists) and Stevie contributed 14 more (plus assists) last season. Trying to replace that 44 goals (and assists) would be a major task in one Summer, a Masherano type figure in front of the back four would help our defence but castrate our attack. Until we have midfielders and attackers who don't need the major assistance of SG then he's irreplaceable in some ways. It's not all about the defence and IMO that's not why he's playing in the DM position even if he isn't a DM. It's to give him that space he needs to do what Coutinho does, but from deeper, and at which he is so brilliant.
I believe that BR has to take this season to revamp and reorganise the forwards and attacking midfielders before turning his attention to DM. Hence the reason he's went after Lallana / Markovic and a Remy/Cavani/Falcao.
I do wish those Alex Song rumours were true though.
The large majority of Gerrard's goals were penalties, so they should be replaceable.