I need a doctor!
Partitive strikes again. Why partitive?
If you google for “Tarvitsen lääkäriä” vs. “Tarvitsen lääkärin”, the latter has twice the amount of hits.
Also, there’s this phrasebook entry on wiktionary:
https://en.wiktionary.org/wiki/tarvitsen_lääkärin
Are both valid, but have different meaning?
I need a doctor!
The reason it is in partitive, is because in this case the individual is asking/pleading for any doctor who may be available due to an unspecified reason and unspecified outcome. They just need a doctor.
If you say “Tarvitsen lääkärin” it is akin to saying “I need the (specific) doctor.” or “I need the doctor’s (ex. signature)”
Hope that helped a little bit.