But that's the thing: you're not tweaking his motivation, just his method. He's going to naturally become bigger and stronger because that is what undertaking AFL preseasons does to you. He's going to put on muscle and he's going to be stronger year on year until he reaches a peak, and when he gets there he's going to be able to break tackles/give off handballs he can't now.
His excellent year under Teague was more to do (IMO) with how Teague empowered certain players rather than used a hard gameplan. Certain players were given licences to do whatever they could in the moment to try and elevate their team (Walsh, Cripps, Docherty, Jones, Weitering, Martin, Betts) where everyone else's roles were to support these players. Get the ball to the movers and shakers, then they drag us over the line. This had a positive effect on some players (Walsh, Jones, Weitering, Martin) and had a negative effect on others (Curnow, Dow, O'Brien, Cripps, Docherty). Under Teague, Walsh flew because he was told he was the only thing between victory and loss. He was told that if he succeeded we succeeded; take every shot, win every ball, execute every kick. Carry the team on your back. As you say, his motivation is exceptional.
Now, the next question is, if it's that simple why not tell him that every time he goes out there? It's pretty ******* clear what happens when you do that: Cripps' back issues, Walsh's injuries since. Don't put it all on player's shoulders, and they won't kill themselves to try and be the difference. Motivation to win is one thing; treating such as life and death is quite another.
The difference is method rather than motivation. Change his method to suit where he will be - don't shove a square peg in a round hole, but make each player grow as a season progresses - as opposed to where he is now.
As you say, Walsh is already - arguably - our best player. Let's see if he can get even better.
Sorry, you lose me when making up a version of what Walsh was told by the previous coach and then trying to link that with an injury sustained the next February in match simulation.
I could just as easily state that the reason for the preseason injury was because they tried to change the way he was to play. That wouldn't be right but it's no less feasible than your scenario.
And for what it's worth Walsh will get better regardless of coaching instructions. That's just Walsh.