To significantly strengthen, we're realistically looking at doing it over a few transfer windows, given how much clubs will be demanding for players.
Therefore, I think in January, the 2 key positions that we need to strengthen are up front (2nd striker), and a creative player in central midfield. I think we should be looking to bring in a player like Mata and then
someone like Ighalo as a backup for Lukaku. I say someone like... because it's Koeman's job to spot the right player and i'm going off Ighalo's record last season. He doesn't seem to have played or scored much this season so it could be an awful shout
. I would sooner Charlie Austin but I see no reason why he'd leave Southampton for us, so Ighalo would be more likely. Having said that, if we're still within touching distance of 6th or 7th in January, I see no reason why we can't be going for a better calibre of player given that there is a chance of European football.
Then in the summer, we need to bring in a top keeper, and centre half. Van Dijk seems the obvious choice at centre half, and the fact he's played under Koeman could lure him here, but realistically again, if the RS or Man United go in for him, we stand no chance. Would also go for Sigurdsson in the summer if Swansea are relgated, and then it will be time to spend the £60-£70m we get for Lukaku on 2 good strikers.