Here is another opinion from another Harvard professor, Lawrence Tribe.
Laurence Tribe on MSNBC talking about DT & whether he could be indicted. - Democratic Underground

Tribe also said the same thing in MSNBC.

FBI agents are impartial. When Mueller calls Trump to testify, he is going to take Trump to school. If it becomes a question of who do you believe Trump or Comey, that is a no brainer, Comey.