The tie is effectively over as the Reds scored five away goals in the 5-0 win in Portugal. But Klopp has insisted he will play his strongest team at Anfield, despite having to face Manchester United at Old Trafford on Saturday.
And the German believes the performance in tonight's match will be an indication on how far the club has progressed in recent times.
"This is a fantastic test for us, a big challenge, because it doesn't just test our ability, our skill, our fitness; it will test the most critical attribute of any side that wants to be successful – our mentality." Klopp wrote in 'This is Anfield'.
"How all of us – players, staff and supporters – approach tonight will tell me more about where we are in our development that any Premier League game against a Manchester City, United, Spurs and so on. We are in a positive way in this moment, but these are the moments of greatest danger.
"These are the moments when you can really learn about an individual's mind-set. If I was in the FC Porto side this evening I would be looking forward to this match more than any other of my professional career.
"I would see the opportunity that is there. So, we must do more than simply mirror the passion and desire they will show. We must surpass it, we must be at a higher level – players, staff and supporters.
"We must prove tonight we want it more than they do; show that we are ruthless and relentless, on the pitch, in the dugout and in the stands; be together as one in this approach."
Klopp added that with all his squad fit and healthy, competition for places has started to take shape within the squad.
And the 50-year-old insists this will also be a key factor if they are to be successful in the future.
He added: "What I have really liked, since the first leg concluded, is that I have seen in the players an understanding that there is a job to be done and that they know they cannot ease off the accelerator for even one second.
"The healthy competition within our own side makes this the case. Never mind opponents, the desire not to be left behind within our own group means they keep reaching and reaching for the next level.
"Take the team selection and squad selection for the weekend just gone, against Newcastle, as an example for what I mean in this moment.
"Can I explain to Simon Mignolet why he doesn't start, based on his own level of performance? No! Or to Danny Ward why he can't even make the bench? Can I tell Joel Matip, Joe Gomez or Alberto Moreno that their level of application and ability doesn't warrant a starting position? I cannot! For Ragnar Klavan, who performs to an outstanding level, I am unable to find a subs spot in this moment.
"This applies right through the team and squad. Lallana, Milner, Solanke, Ings and so on and so on, just using the last weekend as an example. These lads are pushing each other to perform at their maximum every single day.
"It is why, as a manager and as coaches, picking the team and the squad is both the easiest and hardest task of the week.
"Easiest because I have total faith in every one of the team that when given the opportunity to play, they take it and make the most of it. But it is also the hardest task because all these players deserve to be playing – not one has given me an excuse or a reason to say "Sorry, you haven't earned it for today.
"If we continue in this way, with the entire squad displaying the ability and attitude that means they should start every game, we have a real chance of being successful together."