Sanches swapped Portuguese giants Benfica for Bayern in the off-season and the 19-year-old is poised to make his Bundesliga debut against Schalke on Friday.
And as fans prepare to get a glimpse of the Portugal star – a Euro 2016 winner – Alonso heaped praised on the youngster.
"He will be a very important player. He has a lot of energy, he is very strong [physically] and he is hungry. Hungry to win every title. He is a very good signing for the club," said the Spaniard.
"His age doesn't matter. He is 19, but he is ready - he is a top player already.
"That's the most important thing for every player - if he is 19 or 35 years old."
Sanches has not played since aggravating a thigh injury last month, missing Bayern's season-opening 6-0 victory over Werder Bremen, but Alonso said: "I think Renato is feeling very good.
"Maybe there is a possibility for him to play on Friday [against Schalke]. Maybe, but I don't know.
"You have to ask Carlo Ancelotti tomorrow [at the news conference]. Renato trained two weeks very good. He is ready."
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