Champions League Matchday 4: 5 Key Matchups to Watch

Halfway through the UEFA Champions League group stage, the contenders and pretenders are starting to sort themselves out.

Sixteen more matches are on the schedule this week, with eight each on Tuesday and Wednesday. By Wednesday evening, we should have a fuller picture of what the knockout phase will look like.

With so much on the line, vital matchups are sure to abound all over the continent. Here are five of the most important in this week's fixtures.

(All matches are scheduled to start at 2:45 p.m. ET.)

The Left Side of Real Madrid's Defense vs. Borussia Dortmund's Attack

Match: Real Madrid vs. Borussia Dortmund, Group D, Tuesday

German champions Borussia Dortmund defeated Spanish champions Real Madrid 2-1 two weeks ago in Germany in what was probably the marquee game of Matchday 3. The heavy hitters meet again Tuesday in Madrid, and the stakes remain high.

First place in the so-called Group of Death is on the line, and victory would prove valuable for either team's hopes of reaching the knockout phase.

Two weeks ago, Jurgen Klopp's Dortmund found success against the right side of Real Madrid's defense. With Marcelo, Fabio Coentrao and Alvaro Arbeloa out injured, Real manager Jose Mourinho had to play Michael Essien at left-back.

Put simply, Dortmund overwhelmed Essien, who is not a natural left-back. Marco Reus and Mario Gotze flooded the right, taking advantage of the space between Essien and Cristiano Ronaldo.

Not coincidentally, the winning goal came from that area, with Gotze starting the move that led to Marcel Schmelzer's goal with a cross from Real's left side.

Arbeloa returned to Real's lineup at the weekend and played left-back as Real Madrid routed Real Zaragoza. His presence should help stabilize the defense, but will it be enough to deal with Dortmund's offensive threat?

Ramires vs. Fernandinho

Match: Chelsea vs. Shakhtar Donetsk, Group E, Wednesday

Shakhtar Donetsk defeated defending European champions Chelsea 2-1 two weeks ago to take control of Group E. The same teams meet again Wednesday in London, and if both managers pick similar teams, we could have a tasty rematch of Brazilians.

Shakhtar's Fernandinho and Chelsea's Ramires found themselves locked in an interesting head-to-head battle. ZonalMarking.net explains:

"

Once (Ramires) had moved back into the deep midfield role, the two men were in direct combat—and are very similar players: the same nationality, the same height, the same build, the same number, the same stamina—and the same role in this game.

They had the responsibility of connecting the side through sudden bursts into attack, a crucial role in a transition-based game featuring lots of space in midfield.

Both players had good games. Ramires produced a fine example of midfield energy when breaking forward to create a half-chance for Torres towards the end of the first half, but Fernandinho emerged as the game’s key player in the second half.

"

Fernandinho turned in a man-of-the-match performance and played a starring role in Shakhtar's second goal, which turned out to be the winner. Playing away from home will be more difficult, but both Fernandinho and Shakhtar have already demonstrated how dangerous they are in this competition.

Jefferson Farfan vs. Arsenal's Left Side

Match: Schalke vs. Arsenal, Group B, Tuesday

Arsenal fell out of first place in Group B after performing poorly in a 2-0 home loss to Schalke two weeks ago. The Germans outplayed Arsenal at the Emirates and had extensive success attacking the Gunners' left side.

That's where Brazilian full back Andre Santos has been filling in recently during the injury-enforced absence of Kieran Gibbs. Besides angering Arsenal fans with his shirt-swapping antics at halftime against Manchester United on Saturday, Santos has played poorly.

Very poorly.

Schalke identified Santos as a weak link and exploited Arsenal's vulnerability down the left. Manchester United noticed and had similar success, with Antonio Valencia creating width in the not-as-close-as-it-sounds 2-1 victory over Arsenal at the weekend.

Arsenal's recent struggles haven't all been Santos' fault, though. Lukas Podolski has shown either an inability or an unwillingness to cover down the left flank. And central defender and captain Thomas Vermaelen made an error that directly led to United's first goal.

Arsenal's defense, in other words, has plenty of issues. But Santos continues to be the weakest link in a rickety, rusty chain-link fence.

Jefferson Farfan ran riot at the Emirates two weeks ago. If Gibbs can't recover in time to play in Tuesday's match (and if Arsene Wenger again refuses to play anyone else, like Laurent Koscielny, there), expect a repeat in Germany at Santos' expense.

Roberto Mancini vs. Frank De Boer

Match: Manchester City vs. Ajax, Group D, Tuesday

English champions Manchester City are winless and last in Group D. A 3-1 loss to Ajax two weeks ago in Holland left Roberto Mancini's men with just one point in the so-called Group of Death.

The same teams meet again Tuesday at the Etihad, and victory is crucial to City's hopes of qualification.

If City are to win, Mancini must perform better than he did two weeks ago. Mancini was thoroughly out-managed by Ajax's Frank De Boer that night, as B/R Featured Columnist Joe Tansey explains here.

Mancini's experiments with three-man defenses have cost City points this season. Another tactical misstep could send City out of the competition early.

Celtic's Upset Bid vs. Barcelona's Injury-Hit Defense

Match: Celtic vs. Barcelona, Group G, Wednesday

Scottish champions Celtic nearly pulled off a shock two weeks ago at Camp Nou, but Barcelona pulled off a dramatic 2-1 win thanks to defender Jordi Alba's stoppage-time winner.

Celtic had taken a first-half lead in that match through Georgios Samaras, who converted a header from a set piece against Barcelona's injury-ravaged defense. Two weeks later, the storylines remain pretty much the same for both teams.

Barcelona's defense is still thin, but central defender Gerard Pique is reportedly fit and set for the match (via Scottish Daily Record). Pique, however, has not played since Sept. 19 and might be rusty.

Celtic, meanwhile, will probably take a similar approach as two weeks ago, relying on set pieces to attack and defending in organized, disciplined numbers to combat Barcelona's intricate passing game.

The strategy almost worked last time. This time, can Celtic secure a result on home soil?

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