The Atlantic reports Trump is getting antsy about Michael Flynn. You will recall the bizarre questioning of Comey by the Senate where he was asked about witnesses cleaning up their act like registering as a foreign agent or paying back taxes so as to be credible for juries. Comey said that was the process for a witness to survive challenges from a defense.
In short, Flynn has registered retroactively as an agent of Turkey but Paul Manafort has not.
Trump's lawyers have multiple times stopped him from calling Michael Flynn to ask him what he will testify. The suggestion by the stories is that Flynn will be a state's witness. and may have received immunity.