If he does play in Gelsenkirchen, Sanches will need to grab the opportunity to impress Ancelotti with both hands.
The competition for places is intense with Vidal, battle-hardened former Spain midfielder Xabi Alonso and Thiago Alcantara all ahead of Sanches in the pecking order.
Sanches, who was unavailable for the start of the season because of a thigh injury, could go the way of Austria full back David Alaba who forced his way into the team as an 18-year-old and has not looked back since.
Other youngsters have also been successful such as Joshua Kimmich and Kingsley Coman who broke into the team last year.
But there are less encouraging examples such as Switzerland winger Xherdan Shaquiri and Germany forward Mario Goetze, who both arrived amid great fanfare but slunk out through the back door after failing to establish themselves.
It was all very different for Sanches at previous club Benfica, who sold him to the Bavarians for around 35 million euros (29.64 million pounds).
Sanches made his professional debut only last October but showed a maturity beyond his years as he stamped his authority on the midfield, helping Benfica bounce back from a slow start to win the Portuguese league.
His fate will be important for the Portuguese national side where he provided added bite to the midfield during Euro 2016 after being drafted into the team for the quarter-final against Poland, scoring in the 1-1 draw which Portugal won on penalties.
He was sorely missed when the Portugal lost 2-0 to Switzerland on Tuesday in their opening World Cup qualifier.
Portugal coach Fernando Santos would not comment on whether he felt that Sanches had made the right move in joining Bayern, or whether he should have stayed at Benfica where he was sure of regular football.
Santos said that it had nothing to do with him whether Sanches got into the Bayern team.
But it is hard to imagine that Santos will be happy if Sanches spends much of the season warning the substitutes' bench, denying him experience and match practice.
(Writing by Brian Homewood; editing by Clare Lovell)
