It's got to be Medical/Health Facilities along with Academy Training Facilities, this will give you Health Packs to repair players injuries, along with a crop of young players to develop (Saves having to buy players)

Don't throw any tokens/money at your stadium yet, when you first start you just don't need the extra seats

The first stadium upgrade to do is the Pitch itself as this will help to cut down on injuries

If your accessing the game on iPhone or iPad then make use of watching videos to earn Green & Red Packs (don't bother with Blue Packs) and when on laptop/pc watch videos to earn Tokens.

HTH