Can, who marked his first start for Dortmund with a stunning long-range goal, said they only had themselves to blame after conceding twice in the dying stages to stay third, three points behind leaders Bayern Munich.
“When we take the lead, we have to be a little dirtier,” said the midfielder, who joined last month from Juventus.
“That’s something the team has to learn, defensively we all have to be better together.
“It was definitely a setback, but anything can still happen.”
Dortmund were 3-2 up going into the final 10 minutes thanks to goals by Mats Hummels, Can and Raphael Guerreiro.
However after Kevin Volland netted two first-half goals for Leverkusen, substitute Leon Bailey swept home to make it 3-3 on 81 minutes before playing a part in Lars Bender’s headed winner.
There was just 82 seconds between Leverkusen’s final two goals as the Dortmund defence was caught napping.
It was the first time new striker Erling Braut Haaland had failed to score for his new club after hitting eight goals in his first four games, and the teenage sensation cut a dejected figure as he trudged off.
Dortmund will finish the weekend six points behind Bayern Munich if the champions beat second-placed RB Leipzig at home on Sunday.
“That hurts,” said Dortmund’s sporting director Michael Zorc.
“It’s very disappointing because we played well until the 80th minute.
“We’re conceding too many goals, we’re not in a better position (in the league) and we have these (defensive) problems.”
Dortmund head coach Lucien Favre could not hide his frustration.
The Swiss broke off a post-match TV interview when asked about the table situation, “stop taking about it”.
Leverkusen coach Peter Bosz enjoyed some sweet revenge over his former club having been sacked by Dortmund after just six months in charge in 2017.
“I just told the boys they did really well and never gave up,” said the Dutchman, who recently signed a contract extension until June 2022.
