Stage is set for knockout action in Champions League

Real Madrid v Manchester United Echoes, inevitably, of the epic, 6-5 on aggregate, quarter-final of a decade ago, when the other Ronaldo, the great Brazilian striker, with his hat-trick for Madrid in Manchester, earned a standing ovation. The Portuguese Cristiano Ronaldo of Madrid should get a good reception from his old loyalists for the second leg, by which time he will hope to have established a winning platform for the Spanish champions. If not, Madrid will be fearful. They are not having a vintage season domestically, and the long, 11 year gap since their last Champions League title is becoming a burdensome weight. Jose Mourinho, the Madrid head coach, and Sir Alex Ferguson have shared many confrontations - and convivial moments - and the tie is bound to be interpreted as an audition for Mourinho's next big job, very possibly in English football. AC Milan v Barcelona These garlanded clubs met twice last season, in the group phase and in a last-eight tie, and Milan managed plenty of goals: five in all, capitalising on uncertainties in the Barcelona defence against set pieces and swift counter-attacks. Milan may be weaker, on paper, since then, and Barcelona at least as strong as they were, but the Catalan club's wobbliness against Celtic in the group phase shows that for all their unstoppable swagger in the Primera Liga, the competition favourites have one or two flaws. They are two potentially emotional nights for Bojan Krkic, the young striker on loan at the Italian club from Barcelona, where he grew up in the academy. He will be keen to secure a starting berth in the Milan team by then, although the form of Serie A's top goalscorer Stephan El Sharaawy makes that a tough challenge. Celtic v Juventus Celtic must search hard for encouraging precedents, up against the Serie A leaders and title holders. But there are some. Juventus were held to a draw in the group stage by the Danes of Nordsjaelland, novices to the competition, and while the Celtic head coach Neil Lennon is rightly reluctant to hark back too long on his team's November win over Barcelona, Celtic have held their nerve, and played to their strengths when it has mattered in Europe. If a fit Victor Wanyama polices Andrea Pirlo closely, Celtic can make life uncomfortable for Juve, who will hope the influential Kwadwo Asamoah is not still involved in the African Nations Cup final on the Sunday before the first leg. Juve also have tough-looking league matches in the weekends - against Fiorentina and at Napoli - ahead of both legs. Porto v Malaga In spite of losing strikers of the calibre of Radamel Falcao and Hulk in the last two summer transfer windows, Porto's knack of regenerating their firepower, often via astute raids in the South American market, continues. The 2004 European champions are seasoned competitors in the knockout stages of this tournament, too, and this their seventh time in the last 16 in the last 10 years. Malaga, by contrast, are debutants in Europe's most prestigious cup, but so far very impressive ones. They were unbeaten in their group and finished above AC Milan in it, their top placing earning them the right to play the second leg of their last-16 tie at home. The Malaga head coach Manuel Pellegrini took Villarreal to a Champions League semi-final in 2006. He will know his current team now have a decent chance of at least a quarter-final. Valencia v Paris Saint-Germain Another intriguing tie between a club with plenty of Champions League know-how, and the richly, recently assembled squad of Qatar-backed Paris Saint-Germain. Valencia dismissed their head coach, Mauricio Pellegrini at the beginning of this month despite him having guided the club to the last 16 of the Champions League, an improvement on the group-phase elimination that his predecessor, Unai Emery, suffered a year ago. The new man in charge, Ernesto Valverde, calls the meeting with PSG "a tough but attractive tie. They are a side with some great individuals." What PSG, under the two time Champions League-winning coach Carlo Ancelotti, have yet to establish is that those great individuals can combine in a consistently heavyweight unit. They remain very dependent on the goals and creativity of Zlatan Ibrahimovic. Arsenal v Bayern Munich A daunting prospect for the London club. Arsenal took only one point from a possible six against Schalke 04 in the group phase. That is the same Schalke who are currently six places and 17 points poorer than Bayern Munich in the Bundesliga table. Lukas Podolski has points to prove in these fixtures. The German striker, so prolific at international level, and at his two spells with Cologne, spent three mixed years at Bayern. He found himself used as an impact substitute more often than he appreciated. Bayern have not exactly missed him. His replacement, Mario Gomez, has scored 98 goals in 150 Bayern matches over the last three-and-a-half years. With Gomez now back from injury, Mario Mandzukic scoring freely and Franck Ribery in strong form, Bayern are potent up front. But the tussle to control midfield between the two Basques, Arsenal's Mikel Arteta and Bayern's Javi Martinez could be the key. Galatasaray v Schalke Schalke had better hope they recover some morale, urgency and confidence over the Bundesliga winter break. The Germans, impressive in the group phase of the Champions League, which featured an away win at Arsenal, have been slumping badly in their domestic championship. They dismissed their head coach Huub Stevens last week. Schalke were surprise semi-finalists in 2011, but their interim coach Jens Keller needs to refocus his squad if they are emulate that achievement. Galatasaray meanwhile are clear at the top of the Turkish top flight and although they began their Champions League campaign poorly, taking just one point from their first three matches, they recovered well to grab their place in the last 16 of Europe's elite. Burak Yilmaz's goals were crucial in that, and only Real Madrid's Cristiano Ronaldo has scored as many times - six - as him in the competition so far. Shakhtar Donetsk v Borussia Dortmund Arguably the two revelations of the group phase. Or at least a revelation to anybody who though Shakhtar's appearance in the quarter-finals of the Champions League in 2010/11 was a one-off lucky break and had not been following their smooth progress to domination of the of the Ukrainian league; and a revelation to those who judged Dortmund's European calibre on the basis of the German champions' poor showing in the 2011/12 group phase of this competition. Dortmund, who play with terrific energy, have been a different proposition this time. They emerged top of a formidable first-phase group, including Real Madrid, Manchester City and Ajax. They beat Madrid and helped consign City to bottom place. But Shakhtar, fresh from a long break between domestic seasons, may well present an even stiffer test to Dortmund than the Spanish champions, the English champions and the Dutch champions did. Follow us

Stage is set for knockout action in Champions League

As the last 16 get pitted in the elimination stage, Ian Hawkey looks at the clashes and the subplots to watch out for in the new year.