I don't buy this idea that he's key to how we play at all. He does what he is good at, which is mobility and energy. The role he plays is pretty much be where the ball is, and while noone can deny his stamina and and ability to get around the pitch, that can create its own problems.
If the rest of the team doesn't fill space he leaves, or press as fast and aggressively as he does, there's no balance. Think about a team of rowers. If 1 of the team can and does row faster than everyone else, then no matter what numbers he puts up, that boat is going nowhere fast.
People talk about him being this super DM. But honestly now, he's been in England for 4 years now, and while his personal stats have been outstanding, he hasn't been part of any side that has been anything more than average and inconsistent. Whether that's his fault, or the other players, or the managers, etc, is up for debate, but we're not selling someone who has made us better in the time he's been here. He is a decent, hard working, energetic player, and seems like a nice bloke. I wish him well, but I will be extremely disappointed if we can't improve without him, and for the money being quoted, we would be absolute fools to turn it down. We have sold much better players than him for much less.