Lewandowski is without a goal in four European matches after firing a blank in last week's 2-1 first-leg defeat, but Heynckes insists he has no intention of dropping a player who has rattled off 39 goals in 44 appearances in all competitions in 2017/18.
"Lewy has my complete support," Heynckes said at his customary pre-match press conference on Monday. "Every striker goes through phases where they don't score. He once scored four goals in one game against Madrid for Borussia Dortmund. Who knows what will happen tomorrow?"
Watch: What makes Lewandowski so good?
With Lewandowski a confirmed Bayern starter in the lone striker's role, the Poland captain's nominal understudy Sandro Wagner will once again begin the contest on the bench.
The 30-year-old was an unused substitute in the first leg, but goes into the return fixture fresh from scoring his eighth Bundesliga goal in 12 matches since rejoining his boyhood club from Hoffenheim at the turn of the year.
"Wagner played well against Frankfurt at the weekend," Heynckes continued. "He scored, he's useful, he moves very well and could be a real weapon for us during the game. He might come on later on and play alongside Lewandowski. We'll see."
Heynckes otherwise gave nothing away, save Bayern's plan to score the necessary number of goals required - a minimum of two - to rescue the tie and reach the final.
"We need to attack," he affirmed. "We always want to score - that's the Bayern way - but we need to be more clinical than we were in the first leg and we need to take our chances like they did.
"The Juventus game [Juve won 3-1 in Madrid after losing 3-0 in Turin] showed that Real are beatable at home. It'll be difficult for us, but it'll be tough for them, too. We need to be switched on for 90-plus minutes. My team knows how to rise to the occasion."