Bayern Munich have won their last six Bundesliga matches without conceding a single goal - scoring 21 in the process - but Heynckes believes that is nothing compared to the strength Tuesday's opponents in the Champions League.
"It is going to be a real challenge for us," Heynckes said.
"Manchester City are currently on a par with Manchester United, and that says enough really.
"They are a top European team. I am very impressed by them."
If City are to illustrate this presumed superiority on Tuesday night, they will have to do what nobody else has managed since August 7.
Bayern goalkeeper Manuel Neuer has not conceded a goal in 838 minutes since that date, but he recognises how big an achievement it would be to prevent Roberto Mancini's men from putting one past him.
"No team has really put our defence under pressure yet, but I expect them to test us," Neuer said.
Nevertheless, the Germany international, who was just one game away from the final with his former club Schalke last season, hopes to get a positive result on Tuesday night.
"It is important that we pick something up as that would take us a huge step forwards," he said at Monday's press conference in Munich.
Bayern's in-form winger Franck Ribery is confident they can do just that.
He is looking forward to the encounter in front of a full-house crowd of 61,000 at the Allianz Arena, claiming his side need fear nobody.
"Only Barcelona are a great team, but we are getting close to them," he told Germany's Kicker magazine.
"We are not too far away and anything is possible in one or two games.
"Since I arrived here, I have never seen as strong a Bayern team as the one we have now. This is the best Bayern that I know.
"If we carry on playing the way we are, then nobody can stop us.
"I want to win the Champions League with Bayern. I am hungry, like I was when I first arrived here. I am hungry for everything."
Ribery's place in Heynckes' starting XI is not in any doubt, but the Bayern coach does have a few issues to resolve before kick-off.
Defender Holger Badstuber is struggling with flu, while Arjen Robben may not be ready to start yet after playing his first 10 minutes in over a month in the 3-0 win over Bayer Leverkusen at the weekend.
Heynckes will leave it late before deciding on their inclusion, with former City defender Daniel van Buyten an alternative for the former and Toni Kroos likely to keep his place instead of the Dutchman.
Source: Team Talk
Source: Team Talk