Ronaldo's reunion with Manchester United headlines round of 16

Jose Mourniho is under immense pressure now that Real Madrid will not win the La Liga title.

Juan Carlos Hidalgo/ZumaPress.com

The joy of having the Champions League Round of 16 draw in mid-December is that we can spend two months looking forward to the matches that have finally come round this week. For clubs, though, it can be double-egded; those that were playing well then might have had a wobble and be in poor form now (like Schalke); or those struggling then have had time to turn things round and improve (like Valencia). This week's ties are all compelling affairs. Our rundown is below, with one game standing out as the pick of the week:

Real Madrid v Manchester United (Wednesday):

Where to start with the sub-plots surrounding this game? Cristiano Ronaldo faces his former club for the first time since leaving in 2009; Jose Mourinho up against Sir Alex Ferguson, against whom he first made his name in this competition for Porto; or Robin van Persie single-handedly inspiring United this season. Van Persie is one player capable of producing an away goal out of nothing, which would suit United perfectly.

As usual in Madrid, all eyes will be on Mourinho. With the team out the race for La Liga, the twin objectives for the season are winning the Copa del Rey and the Champions League. When Mourinho's side plays well, it destroys teams, as it showed beating Valencia 5-0 last month, and Sevilla 4-1 last weekend; but when off its game, it can be awful, losing 1-0 at Granada ten days ago.

Madrid is without captain and goalkeeper Iker Casillas, a huge miss, even if he has fallen out with his coach; Marcelo has been accused of being overweight and Gonzalo Higuain is coming back off an injury. Last season, when chasing goals, Mourinho would bring on the Argentine to play alongside Karim Benzema: this season they have only played together five times. And yet at least Mourinho knows his best eleven, and his tactical plan: the spine of Segio Ramos-Xabi Alonso-Mesut Özil with Ronaldo breaking in from the left is among the best in the tournament.

Ferguson has many more choices to make: Antonio Valencia on the right wing to track back and stop Ronaldo? Paul Scholes to track Özil and stop the playmaker's passes? Nani to play on the left in the hope that he can magic up a goal from somewhere? These first-leg ties are normally very cagey affairs but with Ferguson desperate for that away goal, this one might provide a surprise. When the draw was made, Real Madrid was the clear favorite: right now, though, there is probably nothing between the teams.

Celtic v Juventus (Tuesday):

The Italian champion is strong favorite to beat its Scottish counterpart but don't underestimate what Celtic has shown in Europe this season: a first ever away win in the competition at Spartak Moscow was followed by a thoroughly deserved home victory against Barcelona. "We are not just a kick and rush team," coach Neil Lennon told So Foot last week. "We detected some weaknesses, stuck to our game-plan, and have some very good footballers who were able to execute it."

His best two players, Victor Wanyama and Gary Hooper, have stayed at the club despite January offers, and the atmosphere at Celtic Park on European nights is legendary. Juventus may have the individual superiority but it has problems too: Giorgio Chiellini is missing from defense and Kwadwo Asamoah, back from disappointment with Ghana at the African Cup of Nations, might not be ready. It also lacks an dependable goal scorer, and the loan signing of Nicolas Anelka may not solve that issue. Celtic will target set-pieces as real scoring opportunities, and while it could make life difficult in Glasgow, the Italian side should edge it into the last eight.

Valencia v Paris Saint-Germain (Tuesday):

Ah, the Beckham effect: not just the sound of club shop tills ringing to the sound of Beckham 32 shirt being sold in the Paris region, but the cheer of PSG fans as the club finally finds some form this season. In fact, since Beckham joined just under a fortnight ago, PSG has put in two impressive performances, beating Toulouse 4-0 and Bastia 3-1 to extend its lead at the top of Ligue 1. That had nothing to do with Becks -- he starts training on Wednesday, the day after this game -- but more to do with Lucas Moura, at ?40m January's most expensive transfer, and Zlatan Ibrahimovic continuing his outrageous scoring form with 21 goals in as many games.

Valencia is also a side on the up: It has recorded last-minute winners against Deportivo la Coruña and Celta Vigo along with an impressive 1-1 draw with Barcelona. New coach Ernesto Valverde is clearly improving confidence among the side. With Thiago Silva and Thiago Motta missing for the visitors, Valencia has a chance to take a first-leg lead, but Ibra might just help PSG sneak into the next round.

Shakhtar Donetsk v Borussia Dortmund (Wednesday):

Is this the most attractive match of the round? Both clubs were arguably the surprise stars of the group stage, with Shakhtar finishing ahead of European champion Chelsea while Dortmund topped the group that contained Real Madrid, Ajax and Manchester City. Shakhtar is a hard team to predict. Its last competitive game was on December 5, a 1-0 loss to Juventus on Champions League group stage Matchday Six. Since then the Ukrainian champion has sold playmaker Willian and replaced him with Taicon, who will make his debut on Wednesday.

Dortmund excelled in the second half of last season and in Robert Lewandowski, has a striker who has scored in his last five league games. If it can repeat its first-half performance in its last away game, the recent 3-2 win over Bayern Leverkusen (it was 2-0 up inside ten minutes), it could be a contender to win the tournament. In Europe, though, it's all about timing: Dortmund excelled in the group stage but did it peak too early? We will soon find out.