"I do not think it's imaginable that I go into such a tournament without match practice," Neuer said at an event on Tuesday night.
Neuer, 32, has been sidelined by a fractured foot since September and is running out of time to be fit in time for Germany's opening World Cup match against Mexico in Moscow on June 17.
Heynckes said there was no chance Neuer would play in Bayern's last two games this season -- the final Bundesliga game at home to Stuttgart on Saturday and the German Cup final on May 19 against Eintracht Frankfurt in Berlin.
"Manuel won't play against Stuttgart -- and also not in the German Cup final," Heynckes told Kicker magazine.
That means Neuer can only get the necessary match practice in two games before the World Cup -- Germany's friendlies away to Austria in Klagenfurt on June 2 and at home against Saudi Arabia in Leverkusen on June 8.
Coach Joachim Loew is still expected to include Neuer in his provisional World Cup squad, which will be named next Tuesday.
The goalkeeper will have to work on his fitness at Germany's two-week training camp in northern Italy from May 23 to June 8 if he is to have any chance of playing at Russia 2018.