It's rare that you'll be able to use Physio Boost in the first place.
I haven't got the rules to hand but I think injuries are -4, -8, -16 and -32 (with a -96 for a negative EL injury). If a player gets a -16 random injury then with the +4 at the end of the game means he's -12 at worst.
If it's a negative EL injury then he needs to be at -5 in the game, lose -16 and gain +4 to be able to use the physio boost. This isn't very likely.
So the physio boost will only be used for -32 and -96 injuries and can only be used once on the -32. Lets say he was at -2, injured to -34, gains +4 -> -30. Takes a +8 EL physio boost and also a +8 not playing gain -> -14. He'll miss 6 or 7 games in total even with the boost, and 8 or 9 without.
Physio Boost forum discussion here