Villas-Boas, 34, has seen Chelsea suffer their worst start to a season under Roman Abramovich and is desperately clinging to his job after just five months in charge.
And Nelson warned: "I don't think Abramovich will be patient with him.
"This is a critical week for Villas-Boas. He knows it could be his last at Chelsea and is already prepared for that.
"Abramovich had no patience with Jose and he must regret getting rid of him as it was his biggest mistake at Chelsea.
"If you compare Villas-Boas with Mourinho, it's not the same. I've never met anyone as psychologically strong as Jose.
"Villas-Boas should have stayed at Porto. He had a strong squad, the players were settled and knew what to do. Things happened automatically.