Shawcross came through United's ranks but decided last season he would never force his way into the first team.
He snubbed a new deal and his £1m move has paid off.
The England Under-21 player said: "I was a United fan and still am. It will be the first time that I want them to lose.
"It will be strange that the whole family will be supporting Stoke.
"I'd have loved to have played as a regular in the first team but not many do. It's a very hard place to make it.
"Sir Alex was disappointed. He offered me a good deal but I had to make the decision."