Spurs are back in the last-chance saloon with the second leg in Germany another make-or-break fixture in a poor season, which could result in a lowest-ever Premier League finish.
Speculation around Postecoglou's future has intensified recently and the Australian head coach  will be without Son due to a foot injury, yet the 59-year-old struck a defiant tone on Wednesday with the tie finely-poised after last week's 1-1 draw.
Tottenham successful navigated a last-16 tie with AZ Alkmaar last month despite a deficit after the first leg in the Netherlands and while they arrive at Deutsche Bank Park following four consecutive away defeats, Postecoglou promised to fight "tooth and nail" to book a semi-final berth.
"Sonny didn't travel in the end. He's the only one who misses out," Postecoglou revealed.
"Yeah, look it's a blow but it's consistent with everything else that's happened this year, so it's just another challenge for us to overcome.
"The reason we left him out at the weekend was to give him time to recover because he's been struggling with this foot thing for a couple of weeks now, but he couldn't make it.
"I don't see it (tomorrow) as salvaging the season, I see it as an opportunity to do something special.
"You're at a point where you have got an opportunity to get to the semi-finals of a major competition. Obviously you know the challenge before you and that's the bit you're excited about.
"That has kind of been my message to the players. Irrespective of everything else that has happened this year, we're a game away from the final four of a major competition.
"There's no burden on me, there's no anxiety on me. What I'm sitting here doing is thinking we've got a great opportunity to get to the final four of a major tournament.
"Mate, I'm not going to let that slip by without fighting tooth and nail for it irrespective of what may come the day after."
Spurs' sorry season took another turn for the worse on Sunday when they shipped four goals at Wolves to stay 15th in the Premier League.
Postecoglou was asked outright about his future and responded: "Do I know? No, I've got no idea. We've got a game tomorrow night, but it's not something I need to think about, I don't think.
"What I do know is we have an opportunity to get to the final four of a major tournament and that's where my focus is."
Tottenham defender Micky van de Ven insisted the squad retained belief, saying: "We all still have the trust in the gaffer and still have trust in the way he's playing."
