O terapeuta disse que o descanso faz parte do tratamento, não é preguiça.

In Portuguese, common nouns almost always take an article (o, a, os, as) when you are talking about a specific person or thing.

• O terapeuta = the therapist (a specific one already known in the context)
• Saying just Terapeuta disse... sounds incomplete or telegraphic, like a note or a headline, not a normal sentence.

So O terapeuta disse... is the natural, full way to say “The therapist said…”.

Yes, the article shows grammatical gender:

• o terapeuta = grammatically masculine → usually interpreted as a male therapist
• a terapeuta = grammatically feminine → usually a female therapist

The noun terapeuta itself ends in -a but is epicene (can be masculine or feminine depending on the article and context), so the article is what tells you the gender here.

Portuguese reported speech is more flexible with tenses than English:

• In English you often backshift:
  
  • He said that rest *was part of the treatment.*
• In Portuguese you can backshift (fazia parte), but if what was said is still true now, the present tense is very common and natural:
  • O terapeuta disse que o descanso faz parte do tratamento.
    → The therapist said this, and it is still true that rest is part of the treatment.

So the present faz highlights that this is a general, ongoing truth, not something only true in the past.

Fazer parte (de) is a very common fixed expression meaning “to be part (of), to belong (to)”:

• O descanso faz parte do tratamento.
  = Rest is (an integral) part of the treatment.

You can say:

• O descanso é parte do tratamento. (correct, but slightly less idiomatic)
• O descanso é uma parte do tratamento. (also correct, but sounds more like “one of the parts”, a bit more technical)

The most natural everyday choice in Portuguese for this idea is fazer parte de.

Portuguese almost always contracts the preposition de with the definite article o / a / os / as:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

So grammatically it’s:

• faz parte de o tratamento → spoken and written as faz parte do tratamento.

Saying de o tratamento separately is incorrect in normal Portuguese.

Here descanso is a noun (rest), and in Portuguese, abstract nouns like this are usually used with an article when you’re talking about them in a concrete, specific way:

• O descanso faz parte do tratamento.
  = Rest (as a defined thing in your treatment plan) is part of the treatment.

You could say Descanso faz parte do tratamento, but:

• It sounds more like a headline, slogan, or note (article dropped for stylistic reasons), not like normal flowing speech.

In ordinary speech or writing, O descanso faz parte... is the natural choice.

The subject is understood from context. The sentence really means:

• (Isso) não é preguiça.
  = (That) is not laziness.

In Portuguese, when the subject is obvious (usually isso = that / this referring to what was just mentioned), it’s very common to omit it:

• O descanso faz parte do tratamento, não é preguiça.
  = Rest is part of the treatment; it’s not laziness.

So não é preguiça is a natural shorthand for isso não é preguiça.

Very close, but with a nuance:

• não é preguiça
  = it’s not laziness (identifies what it is / isn’t)
• não é por preguiça
  = it’s not *because of laziness (talks about the *reason/motive)

In the sentence:

• O terapeuta disse que o descanso faz parte do tratamento, não é preguiça.

you could indeed say:

• ..., não é por preguiça.

Both are idiomatic.
não é preguiça is a bit more direct and categorical;
não é por preguiça explicitly denies laziness as the motive.

Yes:

• O descanso faz parte do tratamento.
  = Rest is part of the treatment. (noun)
• Descansar faz parte do tratamento.
  = Resting is part of the treatment. (verb used as a noun → “to rest” / “resting”)

Both are correct and natural.
O descanso focuses slightly more on the thing (the rest periods);
Descansar focuses a bit more on the action of resting. In practice, they’re very close in meaning.

In simplified IPA for European Portuguese:

• O terapeuta disse que o descanso faz parte do tratamento, não é preguiça.
  /u tɨɾɐˈpɐwtɐ ˈdisɨ kju dɨʃˈkɐ̃su faʃ ˈpaɾt(ɨ) du tɾɐtɐˈmẽtu, nɐ̃w ɛ pɾɨˈɡisɐ/

Key points:

• O terapeuta → u tɨɾɐˈpɐwtɐ
• disse → ˈdisɨ
• que o often sounds like kju in connected speech.
• descanso → dɨʃˈkɐ̃su (the s between vowels → [z] or [z]-like, but in d‑ESCAN‑so it’s [s] before consonant; the e in de is very reduced)
• faz → faʃ (final z-sound written as z/s often goes to [ʃ] in EP)
• parte → ˈpaɾt(ɨ) (final e is very reduced or almost gone)
• tratamento → tɾɐtɐˈmẽtu
• preguiça → pɾɨˈɡisɐ (the ç has an s sound).

This is a careful but natural European Portuguese pronunciation.