Which one is correct?

We will follow up the patient with a postop phone call?
or
We will follow up with the patient with a postop phone call?

The second sentence.

Both sentences can be grammatically correct, but the appropriate choice depends on the intended meaning.

1. "We will follow up the patient with a postop phone call."

In this sentence, the noun "patient" is being followed directly by the preposition "up." This construction implies that the action of following up is directed toward the patient. So, if you mean to emphasize that you will follow up with the patient specifically, you can use this sentence structure.

2. "We will follow up with the patient with a postop phone call."

In this sentence, the verb "follow up" is followed by the preposition "with," indicating that you will follow up with something or someone. In this case, the object of the preposition "with" is "the patient" and the manner or method of following up is "a postop phone call." This structure is commonly used when specifying how you will follow up (e.g., via email, in person, etc.). If you want to emphasize the method of follow-up (in this case, a postop phone call), you can use this sentence structure.

Ultimately, both options can be correct; it just depends on what you want to emphasize in your statement.