A little more background . . . captain does not = manager although a playing manager can be both.
It was an honor to be named team captain, typically a player with good skills, deep knowledge of the game, and well liked by players. If the manager happens to also be a player than you could have someone like Anson who was both captain and manager (manager has say in naming the captain).
In 1887, Detroit had Watkins as manager and Hanlon as captain (& center fielder), Dunlap was merely 2nd baseman. Dunlap was heavily recruited by Pittsburgh as the 1887 season came to a close. Dunlap would be signed by Pittsburgh one week after this late 1887 entry from the Pittsburgh Post with indications that they wanted him as captain.
Here is another entry before start of 1888 season in which he is referred to as captain. He would be known as Captain Dunlap his whole time with Pittsburgh. He would manage 17 games near the end of his stay but that had nothing to do with his title as Captain.
Dunlap would have 9 different N172 poses issued by Goodwin & Co. in 1888, all with "Capt. Dunlap" at nameplate.