Madrid currently sit in fourth in the La Liga table, 19 points behind arch-rivals Barcelona and are widely being written off by critics ahead of their last-16 fixture against PSG.
Manager Zinedine Zidane is very much under pressure, with European silverware now their only chance of bringing something back at the end of a disappointing campaign.
But Heynckes, who was Madrid manager during their 1998 Champions League title win in his only season at the Bernabeu, thinks that Real will be able to beat Neymar and co and progress into the quarter finals.
“That Madrid is going through a low in the league at the moment is, in my opinion, quite normal, especially if you have won the Champions League twice and been Spanish champions too,” he told Goal.
“In addition, Real have given away three very good players in the summer with Alvaro Morata, Pepe and James Rodriguez, perhaps to save salaries.
“They have not bought big and thought that with the young players they can compensate for that. You need a really good mix between young and old, between experienced and hungry players.
“Nevertheless, you must not write off Real in the Champions League. They have much more experience than Paris Saint-Germain. Much more! I believe they will win the tie.
Can anyone tell me how Real Madrid won two Champions Leagues but are now barely top 4 quality with essentially the same team?
— Para (@TheParacelsus) February 6, 2018
“They have a good team for playing in European competition. When we first won the Champions League again in 1998, we finished fourth in the league. I know from personal experience Madrid should never be underestimated.”
Heynckes has already faced PSG this season, with Bayern facing the French giants in the group stages earlier in the season.
Bayern haven’t won the top prize in European football since 2013 but are being tipped as this year’s dark horses after Heynckes resurrected their poor start to the campaign under Carlo Ancelotti.