The Constitution lists a handful of minimum requirements for the presidency: age, citizenship status, and residency status. The drafters of the Constitution, not having foreseen the movie Idiocracy, assumed the nation's qualified electors (that means us voters, dummy) would take additional factors into account that speak to a candidate's fundamental fitness to do the job.
Things like, for example, doing due diligence in selecting essential personnel. Avoiding unforced errors like naming an unregistered foreign agent as your campaign manager. That kind of thing.
The departure also comes as Manafort is defending himself from investigations into his extensive lobbying history overseas, particularly in the Ukraine, where he represented pro-Russian interests. Manafort has been beating back reports from multiple media outlets in recent days over his ethics, which have been egged on by a Clinton campaign eager to highlight Trump's ties to the Kremlin.
In fact, Trump's critics in both parties were aware of, and pointing out, Manafort's Russian ties months ago. In characteristic Trump fashion, he made a seat-of-the-pants choice and put a man under a known ethical and legal cloud in charge of his campaign. If Trump's critics knew about it, Trump should have.
Either he didn't -- which speaks terribly of how he'll "hire the best people" -- or he did and disregarded it as an issue -- which says something else entirely about him.
Neither makes a good case for electing him President of the United States.