Underdogs Frankfurt held Chelsea to a 1-1 draw at home in last week’s first leg, but face a fight against the odds at Stamford Bridge if they are to make a first European final in 39 years.
Kovac, who led Frankfurt to German Cup glory last season before taking over at Bayern, said he would be crossing his fingers for his former players.
“I sent a few messages before the first leg, and I will do so again tonight,” he said on Thursday afternoon.
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“I hope they can pull off the necessary miracle. It will be a question of passion and strength of will.”
“You can win as the underdog: we showed that last year.”