A shock 3-1 defeat in Porto last week has left Bayern Munich with no margin for error if they are to avoid a devastating early exit from this season’s Champions League. In Tuesday’s second leg at the Allianz Arena, Bayern will have to win by two clear goals to have a chance of progressing to the final four.

With the Bundesliga title again already all but sewn up, Bayern’s season was once again expected to come down to whether they could produce the goods at the sharp end of the Champions League and make amends for last season’s chastening semifinal defeat to Real Madrid. Few expected Porto, in their first quarterfinal appearance in six years, to throw those plans into such doubt.

Things went about as bad as could be imagined for Bayern in Portugal last Wednesday. Missing the talent of Arjen Robben, Franck Ribery and David Alaba, among others, their attack was left blunted and predictable to a degree rarely seen since Pep Guardiola took charge in the summer of 2013. A lifeless tone permeated a side that was also without Bastian Schweinsteiger, Mehdi Benatia and Javi Martinez. But what made it arguably the worst display of Guardiola’s reign, and has given Bayern a steep hurdle to climb in the second leg, was the contagious sloppiness out on the Estádio do Dragão pitch. All three goals, including two in the opening 10 minutes came from Bayern errors.

The fall out has reflected the scale of the surprise and disappointment. Guardiola’s very future at the club beyond this season has been questioned, while club doctor Hans-Wilhelm Muller-Wohlfahrt quit his role after 38 years at the club, claiming that the medical department had been blamed for the defeat.

It was more than just a story of Bayern’s failings, however. And sizable credit for forcing and exploiting Bayern’s blunders must go to Porto. Coach Julen Lopetegui, a one-time teammate of Guardiola at Barcelona, designed a strategy to perfectly exploit Bayern’s want to push high and pass out of the back. Surely not even Lopetegui, though, could have dreamt of just how successful his plan to press Bayern in frenetic bursts would prove.

Younger talent like Algerian Yacine Brahimi, Mexican Hector Herrera and on-loan Real Madrid midfielder Casemiro often caught the eye as Porto won their group and sauntered past Basel in the Round of 16. But, along with Colombian striker Jackson Martínez, against Bayern it was winger Ricardo Quaresma who stole the show when scoring the first goal from the spot and adding the second soon after. Once thought by many to have more talent than Cristiano Ronaldo as he came through the ranks with the current world player of the year at Sporting Lisbon, the now 31-year-old failed to deliver on that undoubted ability at European giants Barcelona, Inter Milan and Chelsea.  But he again seems at home in Porto, and the way he bought into his coach’s strategy was indicative of a true team performance.

They will be all too aware, though, that the job is only half done. And Porto’s task has been made significantly tougher by suspensions that rule out their two first-choice full-backs, Alex Sandro and Real Madrid-bound Danilo. Still, their opponents remain short of numbers and options. Of those players who missed out last week, only Schweinsteiger is fit to return, while Philipp Lahm and Juan Bernat have had to overcome fresh knocks to take their place in the squad.

“Porto have the fresher legs, but tomorrow it’ll be decided not by legs but by heads,” Guardiola said, reports Bayern Munich’s official website.

Indeed, with so many players unavailable, this will not be a Bayern Munich at their best. Still a 2-0 win is far from being beyond the bounds of possibility, although it will require those who do take the field for the five-time European champions to be far sharper of both mind and body.

Prediction: Guardiola will doubtless have been drilling into his players the need to sharpen their passing all over the pitch, and it is difficult to envisage them being as poor this time around. Yet overcoming the loss of Robben remains a huge obstacle for Bayern, given how key the Dutch winger is to offering a more direct route to accompany the team’s possession style. With width at a premium, the crucial figure now could be Thiago Alcantara, who showed when scoring in the first leg that he provides a more vertical option in central midfield.

And there is reason for optimism for Bayern. Dating back to his Barcelona days, Guardiola’s teams have been much stronger in two-legged playoffs at home. He is certainly not unfamiliar with the pressure of overturning first-leg deficits. Even in this season’s Round of 16, Bayern followed up a goalless draw away from home against Shakhtar Donetsk with a 7-0 win at the Allianz Arena. That won’t happen this time around, but Bayern are capable of getting the win they need.

Predicted score: Bayern Munich over Porto, 2-0