Its probably in addition to Corey’s other duties. It does not seem like a full-time job.
From a recent Weekly Standard article. (OK, it's the Weekly Standard and with a Politico reference to boot. I get it. But it still rings true to me.)
Here it is:
"In charge" is exactly what Manafort appeared to be shortly after his hiring in late March. It wasn't just appearing on the Sunday shows. Manafort also began asserting a more active role in shaping the direction of the campaign and the candidate. Manafort was "gaining influence" and "consolidating his own power," said Politico, while Lewandowski was "losing the tug-of-war" and "has been neutered." New York magazine's Gabriel Sherman reported Manafort had "taken over" the campaign.