Hi - I've wasted many a Health Pack to heal my star trainer only to be left short later on in the season.
Health packs are hard to come by. You can get 1 a day from friends and probably about 1 for every 2 games you play, and about 6 a week from your facilities. So you're looking at about 60 ish a season.
7 or 9 injuries can wipe that out leaving you short at seasons end if you need your best players for finals/league run in.

Now this is my opinion on healing players and how i got to it.
I play a lot of friendlies - they increase players training points faster or equivalent to practice match but you have the bonus of a marginal income, and a potential of 2 morale packs, 1 rest pack and 1 health pack.

I try play about 3 to 4 friendlies at least a day. Depending on fixture congestion.
So i average 6 rest packs a day (matches, facilities, and gifts)
But only 3 health packs.

Injury Length - Health Packs - - - or - - - Potential Training time Lost
1 day - - - - - - - - 5 - - - - - - or - - - - max 40% rest points lost for training.
2 day - - - - - - - - 9 - - - - - - or - - - - 80% rest points lost for training.
3 day - - - - - - - - 12 - - - - - or - - - - 120% rest points lost for training.
4 day - - - - - - - - 14 - - - - - or - - - - 160% rest points lost for training.
+5 day - - - - - - -15 - - - - - or - - - - +200% rest points lost for training.

If you have played the game for a while - a costly long injury to a player can make the difference between him being in your squad next season or not...

But this is what i feel you should do when injuries come knocking (provided you can cope with an absent player with your squad).

1 day injury
One day injuries take 5 health packs. That's the equivalent of a week+ from your hospital facility. Or if you're lucky with no yellow cards the equivalent of 5 matches but more likely 8-10 matches. So for the potential loss of 40% at most (depending on time of injury) it just isnt worth it.

How I handle it. If the player loses 40% of his 3 hourly rest (5x8 = 40%) then Instead of healing him, i will compencate for the training time lost by allocating him 3 rest packs (3 rest packs = 45%).

So the player actually get an additional 5% for training with me spending only 3 rest packs and not 5 health packs.

2 day injury
Again - 9 Health packs for 80% training loss? not woth it when 6 rest packs equal 90% so the player actually gets an additional 10% training time for 6 rest packs vs 9 health packs.

3 day injury
A player loses 120% rest time in 3 days this is the equivalent of 8 Rest packs or alternatively 12 to heal him. So it's still not worth the health packs.

4 day injury
This takes 14 health packs to fix or 11 Rest packs. Players will only start regressing in rest % if they are injured for to long. so it's more likely that you would need 12-13 rest packs.

SO this is where it gets a bit more subjective. If it's a class player i dont want out for THAT long i do this
A) I heal the player as if it were a 3 day injury (costing 12 health packs) - dropping him down to 1 day injury, and then replenishing him with 3 rest packs. Although this cost 15 packs in total (12 health and 3 rest) compared to 14 health packs i feel the 2 extra health packs are worth more then the 3 rest packs.

If it's not that important a player....
B) I heal the player as if it were a 2 day injury (costing 9 health packs) - dropping his injury time to 2 days, and then i replenish him with 6 rest packs. Again the 15 packs in total (9 health and 6 rests) is more than the 14 Health packs needed to heal him straight up. But I feel 5 health packs are worth a lot more then 6 rest packs.

+5 day injury
This i heal straight up. It's not worth having a player out for that long. You could use the process of 9 or 12 health packs to reduce his injury to 2 or 3 days. And the compensating his time out with rest packs. But i just use the 15 straight up and save them for this exact occasion.

NB - when applying the 3, 6, 9 health packs to a player to compensate for time out - I like to spread this out over a period 1 extra 15% (1 rest pack) a day at max 2 rest packs. Training your players to quickly. IE. Using all 9 packs on a player and blasting him with training (power training) is more likely to cause an injury and that completely defeats the point of the excercise...