I want to describe a player who is not officially the captain of a team but shoulders as much responsibility. I have read the phrase in name only being used when a person holds a title, but doesn't perform or is unable to perform the duties typically associated with it. I would guess that in my case I am looking for a phrase which is opposite of in name only. The person is performing the duties but doesn't officially hold the title.
I searched for the opposite on Google but couldn't find one. Ended up with a similar result from a couple of dictionary searches as well.