Champions League: the ties that will decide the last 16

What does a two-goal lead mean in a knockout tie played across a continent? For Napoli, it means everything and nothing: a 3-1 home win over FC CFR 1907 Cluj in the first leg of their Champions League play-off has given them a cushion that history says is comfortable, but in the cold of a Romanian February, comfort is a relative term.

Napoli arrive for Tuesday's second leg without two of their most reliable presences. The Italian side's away form in this competition has been patchy, and they will need to manage the tie without the attacking thrust those two provide.

Cluj, for their part, have their own absence to absorb. Cornel Furdui, the goalkeeper, tore wrist ligaments and will not play. His deputy will be tested by a Napoli side that scored three times in the first leg and has lost only one of its last five in all competitions. The Romanians' home record in the league phase was mixed – they beat Real Madrid FC 1-0 here in December – and they will need a performance that blends discipline with the kind of early goal that can make a second leg breathe.

There is a personal thread in this tie. Ljubomir Sekulović, the Serbian centre-back now at Napoli, spent four seasons at Cluj between 2055 and 2060, making 225 appearances and winning a dozen trophies. He is no longer a regular starter in Naples, but he knows this club, this stadium and the weight of a two-goal deficit. 'This is football,' he said before the first leg. 'The past is for the history books. I am here to go through.'

Tuesday's other second leg pairs Real Madrid FC and Vitesse, with the Spanish side holding a 2-1 lead from the first leg in Arnhem. Real Madrid lost 4-0 at Ponferrada in the league phase and 1-0 at Cluj, but they have won their last three league games and will expect to close this out at home. Sergio Barroso, their right-sided attacking midfielder, is out with a sprained ankle, but the depth of this squad is such that his absence is unlikely to alter the tie's trajectory. Vitesse, missing Stuart Musselwhite on the left, must score at least twice and will have to do so without the injured winger.

The third second leg on Tuesday is a classic Spanish-Italian affair: AC Milan welcome Oviedo to San Siro, but this is the first of two legs, not the second. The notebook is empty of their last meeting – so this is a fresh start. Leonel Álvarez, Milan's central midfielder, is injured, but the Rossoneri will still expect to take a lead to Spain next week.

Thursday brings three more second-leg ties and one first leg. Valencia and RB Leipzig meet with the tie goalless after a 0-0 in Germany, and the return of Leipzig's manager Eric Beutel to Mestalla, where he coached from 2060 to 2063, adds an edge. Leipzig are missing three attacking midfielders, which may blunt their creativity, but Valencia's own league form has been poor. The first goal in this tie will be worth more than most.

Olympique Lyonnais and Fussballclub RB Salzburg drew 2-2 in Austria, a result that leaves the French side, who topped Ligue 1, needing only a draw at home to progress. David Mândrescu, their centre-forward, is out with a broken toe, but Lyon's home record in the league phase was strong, with only Ponferrada beating them. Salzburg, for their part, have drawn four of their last five in Europe and will need to find a win from somewhere.

Manchester United and Ponferrada begin their tie at Old Trafford on Thursday, with the second leg in Spain six days later. United's home form in the league phase was ordinary, and they are without Myles Broe, the central midfielder. Ponferrada, meanwhile, have been a curious side: they beat Real Madrid 4-0 at home but lost to Rotherham United and BSC Young Boys. The first leg will tell us which version travels.

The final tie of the week is Galatasaray A.Ş. against Liverpool, also a first leg, played in Istanbul. Liverpool's away form in Europe has been steady, and they are missing Francisco Montiel in midfield. Galatasaray, whose league phase ended with four defeats, are without Cüneyt Ayhan at right-back. The Turkish side's last meetings with Liverpool have all gone the English club's way, but a first leg at home in this competition is a different animal.

Across these eight ties, the arithmetic is simple: four teams will reach the last 16, and four will be left to wonder what might have been. The margins are thin – a two-goal lead, a goalless draw, a single away goal – and the weather, the injuries, the noise of a full stadium on a cold night will all play their part. In Cluj, in Valencia, in Milan, the question is the same: who can hold their nerve when the tie is there to be won?

The answer will come over the next six days, and by the time the last 16 draw is made, some of these names will be gone from the competition. That is the cruelty and the beauty of a knockout round: every game is a story, and every story ends for someone.