Comma with 'who' in this specific sentence

No, from the logic of the sentence, it appears to be restrictive.

Non-restrictive clauses are oh, by the way. They do not convey essential information about the noun or pronoun they modify. In

The coach came to the defense of the player who has been sacked from the team following his differences with the captain.

the coach did not come to the defense of just any player, but in particular, the player who had been sacked from the team. He went to the defense because the player had been sacked.

If the sentences were

The coach came to the defense of the player, who has a dog named Clyde.

the clause would have been non-restrictive, that is, not essential to the sense of the sentence. Such clauses are often called parenthetical because they could be contained in a grammatical aside (often contained in parentheses).