He is 20 and hasnt really been over played.
Started 15 of 22 league games and 3 of 6 CL games for example.
Last season Alba played a lot of minutes and this season Balde has started majority but not all and also not been involved with Spain.
The number of games recently may have cotributed but thats not so much about his age at 20.
The issue is more about signing likes of Alonso who is in mid 30s and had injury issues, signing Inigo who was injured and had issues, renewing Roberto who is old and had injury issues then in other positions signing Romeu who isnt good enough. There has been less rotation than should have been due those signings really.