Usta će utrnuti posle injekcije.

Because usta is one of those Serbian nouns that is grammatically plural even when it refers to one mouth. In English we translate it as singular: mouth.

So:

• usta = mouth
• but grammatically it behaves like a plural form

That is normal in Serbian. In this sentence, you do not really notice the agreement because the future is formed with će + infinitive.

Here it means mouth.

If you specifically want to say lips, the usual word is usne.

So:

• usta = mouth
• usne = lips

In a dental or medical context, Usta će utrnuti is a natural way to say that the mouth area will go numb.

Serbian often leaves out possessive words like my, your, his, etc. when they are obvious from context, especially with body parts.

So in a dentist’s office, Usta će utrnuti naturally means:

• Your mouth will go numb

This is very common in Serbian. Compare:

• Boli me glava. = My head hurts / I have a headache

Serbian does not need to say moja glava there, and it does not need vaša/tvoja usta here either.

This is the normal Serbian future tense formation:

• će = will
• utrnuti = infinitive, to go numb / to become numb

So:

• će utrnuti = will go numb

More literally, Serbian forms this with the auxiliary from hteti plus the infinitive.

Also, će is a clitic, so it usually comes in second position in the sentence:

• Usta će utrnuti.

You may also see a joined form such as:

• Utrnuće usta.

That also means The mouth will go numb.

Utrnuti means to become numb, to lose feeling, or to go numb.

That is slightly different from simply to be numb. It focuses on the change into that state.

So:

• Usta će utrnuti = The mouth will go numb

In context, English might also say Your mouth will be numb, but the Serbian verb itself suggests becoming numb.

Because this verb does not need se in this pattern.

With body parts, Serbian naturally says things like:

• Ruka mi je utrnula. = My arm went numb.
• Noga će utrnuti. = The leg will go numb.

So Usta će utrnuti is correct without se.

Because the preposition posle requires the genitive case.

The base form is:

• injekcija = injection

Its genitive singular form is:

• injekcije

So:

• posle injekcije = after the injection / after an injection

This is a standard case pattern:

• posle ručka = after lunch
• posle časa = after class

By itself, it can mean either one.

Serbian has no articles, so there is no separate word for the or a/an. Context tells you which meaning is intended.

In this sentence, especially in a medical or dental setting, posle injekcije will usually be understood as:

• after the injection

Yes, Serbian word order is fairly flexible.

The given sentence:

• Usta će utrnuti posle injekcije.

is a neutral, natural order.

You could also say:

• Posle injekcije će usta utrnuti.
• Utrnuće usta posle injekcije.

These mean essentially the same thing, though the emphasis may shift slightly. The important thing is that će behaves like a clitic and usually stays near the second position in the clause.