Baloney.
Same as the grass upgrades decrease the chance of injuries by ??? mine currently 40% and still manage to average 17 for the past two seasons.
Oh just one point that is not mentioned in the guide.
The injury ratios are coded from 'Physiorooms' premier league stats for the season 2010/11 unless they have been updated, that you can take as fact as I was told by you now who.
So no matter what upgrades and how many packs you use, events are random, so random 9out of ten injuries are always your key players.