Which is why its intriguing to see how long we can go on a good run of games. He faces a dilemma if the current system works. Does he get players to fit the current system or players to fit his system like at Southampton? There's a few key differences.
He played 2 dynamic forwards and a striker with Southampton, and his strikers would often share the goals between whilst the forwards chipped in as well. Obviously he felt he would wait to the summer until solve this problem because now we only have one natural wide player fit in Lennon.
We have a natural goal scorer, who is aided by an engine room midfield. Lukaku's future is key to this. If stays, I doubt we'll see a drastic change to the system. If you play two wingers with him in the team you are wasting a players. The team suffers defensively.
The different systems will require different ventures in the transfer market. If we stick with the current 3-5-2, we will need defensive and wing back cover (Keeper is priority regardless), and we won't see many forwards come in other than squad players.
If we revert to Koeman's 4-3-3, we'll see forwards come in, and if Lukaku does leave sooner rather than later we will see 2 or three striking options akin to what he had at Saints (Long, Pelle, Austin).
I have a feeling this is all resting on the Lukaku situation. He scores goals, he can change games. But the uncertainty of his future is also making the future make up of our team uncertain.