A common and classically attested way of saying "to perform the role of X" is munere X fungi, where "X" is an adjective or genitive noun. Here is an example:
fungar enim iam interpretis munere, ne quis me putet fingere. (Cicero, Tusculanae Disputationes, 3.41)
I will now play the role of an interpreter, lest anyone accuse me of making things up.
(Note that vice X fungi is also a common expression, with an important difference in meaning: it means to "assume the role of another" outside the normal course of things.)
My proposed translation of your sentence is thus:
Marcus, munere ducis fungens, locutus est.
I did a quick corpus search and think a short caveat is in order: the present participle of fungens is not very common, probably because it is deponent. Here is one clear attested example though:
Datames militare munus fungens primum, qualis esset, aperuit in bello, quod rex aduersus Cadusios gessit. (Cornelius Nepos, Vitae Dat., 1.2)
As a final note, I recommend praepositus or praefectus as better translations of "manager".
