Kagawa plays exclusively in an advanced midfield role and doesn't fit in a flat midfield four. Lucas possesses the same vice, so it seems evident that Sir Alex Ferguson is considering a change in formation.
Let's take a look at the tactical options open to Fergie using the current squad personnel.
Option 1: 4-2-3-1
The current "in" formation in world football is the 4-2-3-1—a system that exercises control, defensive stability and attacking prowess.
In recruiting a top-tier attacking midfielder, Manchester United would be wise to look at this formation with sincerity. Kagawa flourished in the same one with Borussia Dortmund, while most peripherals in the team can fit into it nicely.
Paul Scholes and Michael Carrick would form a solid holding duo, while Lucas could slot into any of the three advanced midfield slots. He'd be most at home in the middle or on the right, but you can't see Antonio Valencia being displaced.
If Lucas joins Kagawa at United, either one of Nani and Ashley Young will find game time very difficult to come by, as will Anderson as he's not fit for the holding role and too far down the pecking order for the central attacking role.
Wayne Rooney would likely fulfil a lively forward role, but Javier Hernandez could also slot in nicely.
Option 2: 4-4-1-1
If Ferguson remains reluctant to switch to a 4-2-3-1, the 4-4-1-1 utilised last season is still applicable to the current squad.
It allows Kagawa to play in his preferred position while staying relatively close to the 4-4-2 Fergie has trusted for so long.
It may even be more efficient, as Rooney was often asked to fulfil the deep role, thus drawing him further from goal than perhaps United would have liked.
Moura, too, could play the creative role behind the striker, but would be a little restricted on the edge of a flat midfield four. Anderson and Tom Cleverley are more likely to find playing time in this system too.
Wide formations are a must considering the personnel in United's team. They have a selection of plays who can hug the line, yet no true central midfielders with an engine.
For this reason, the likes of Italy's diamond at Euro 2012 is simply not an option for Ferguson this season.
A 4-5-1 is perhaps a final option, as Kagawa and Lucas cutting in off the line would be a sight to behold. Pack in a midfield of Scholes, Carrick and Phil Jones behind them and you've got yourself something solid.
There are plenty of options available to Ferguson this coming season. This season will be the first in a long time he can utilise a world-class attacking midfielder, thus changing the dynamic of his team.
All these options are still available should Lucas not arrive, as Nani or Young will easily fill the vacant slot.