A comma is sometimes used to set off contrasting expressions within a sentence.

• This project will take six months, not six weeks.

When two or more contrasting modifiers or prepositions, one of which is introduced by a conjunction or adverb, apply to a noun that follows immediately, the second is set off by two commas or a single comma, or not set off at all.

A solid, if overly wordy, assessment

• or a solid, if overly wordy assessment
• or a solid if overly wordy assessment