Did he? He only worked for the campaign briefly. He seems irrelevant.
At this point he is an object lesson to potential witnesses against Trump that you had better play ball with Mueller or else you go to jail.
Cruz and his campaign manager, super-cheat Jeff Roe, had a plan to steal delegates right up until and during the convention. Trump hired Manafort to minimize the success of that plan. So in a very real, practical sense, you can thank Cruz and Roe for Manafort’s role in Trump’s campaign.