For all the discussion about Liverpool's electric front three and their less convincing defence this season, Jurgen Klopp's mission at Anfield has essentially been to build a team based around midfield.

It's worth remembering, for example, that Klopp's use of Roberto Firmino up front rather than a proper striker like Christian Benteke was essentially a vote for a No. 10 rather than a No. 9. It was broadly uncontroversial, in keeping with Liverpool fans' general faith in Klopp's vision from the outset, but in a country that has traditionally been sceptical of moves away from using a traditional centre-forward, Klopp's decision paid off excellently. Firmino was an attacking midfielder rather than a forward, but Klopp was confident he would gradually develop the specific talents needed for goal poaching too.

At the back, Liverpool's centre-backs are relatively traditional defenders, but they're most comfortable when using a high defensive line, which is possible because of the aggressive counter-pressing in more advanced positions, which means Liverpool are essentially defending the midfield zone rather than the penalty box. When forced backwards and put under sustained spells of pressure, like in the final 20 minutes against Roma last week, Liverpool can crack. But when allowed to play their desired game plan, Liverpool command the midfield zone from front to back, and therefore make the lives of their midfielders much easier.

That should not, of course, detract from the excellent work of the midfielders themselves, but it's worth considering quite how, and why, Liverpool's midfielders have looked so good in European competition in recent weeks.

Jordan Henderson, widely criticised for his poor form at the start of this season, was outstanding in the 5-2 first-leg victory over Roma, cutting out danger in front of the back four excellently and providing steady distribution to the flanks. He's naturally a box-to-box midfielder rather than a holding player, but now plays that deeper position well, particularly when his teammates press.

James Milner, for so long considered a bit-part player and rarely used in his favoured central role, was arguably Liverpool's best player in the 3-0 thrashing of Manchester City, pressing tirelessly, scrapping wholeheartedly and popping up with a goal against his former side too.

James Milner and Jordan Henderson have excelled in Liverpool's midfield this season. Andrew Powell/Liverpool FC via Getty Images

Alex Oxlade-Chamberlain, a largely straightforward player technically, has also been well suited to Liverpool's tasks in big matches, more so than when Liverpool are playing against deep defences, which remains their major weakness.

At the start of the season, few would have argued that any of these three truly belonged in a midfield trio seriously challenging for the European Cup. At most, you'd expect one playing a functional role alongside two more creative players. But they've played so well individually because the collective plan is evident: the aggressive, counter-pressing approach combined with assistance from defence and attack, minimising the amount of space Liverpool's midfielders must cover. They used to say that hard-working midfielders "covered every blade of grass", but Liverpool's midfielders appear to work so hard precisely because they don't have to, their zone of responsibility is essentially fenced off both in front and behind.

Oxlade-Chamberlain's unfortunate season-ending injury, however, means Klopp now has only three midfielders for three positions. Adam Lallana is also out injured, Philippe Coutinho has departed, Naby Keita hasn't yet arrived. So, for the remainder of the season Georginio Wijnaldum, Milner and Henderson are Klopp's only three midfielders -- and will have to start two crucial games this week, away at Roma and then on Sunday at Stamford Bridge. Suddenly, Liverpool are having to fight for fourth place, with Chelsea on the charge.

The extent of Klopp's problems in midfield are underlined by the fact Trent Alexander-Arnold started in midfield against Stoke on Saturday. The youngster played in that position for the club's youth team, but it's difficult to imagine this being a serious option in a potential final against Real Madrid, for example, and would also mean Liverpool having to turn to Joe Gomez or Nathaniel Clyne at right-back, rather than the player who excelled there against Manchester City in the quarterfinal.

This week, Liverpool will again attempt to dominate the midfield zone with their mobility and fighting spirit, although it will be a different task from the reverse fixture. Roma's midfield will be tilted differently, with Daniele De Rossi in the holding role of a 4-3-3 rather than playing side-by-side with Kevin Strootman, who has been ruled out through injury. Liverpool's players, also playing in a 4-3-3, will probably take it in turns to press him; sometimes it will be Firmino dropping back, sometimes Milner and Wijnaldum when Roma have the ball in full-back positions, and sometimes even Henderson, pushing forward alongside his two midfield colleagues. De Rossi is the man who can get Roma playing good football, as he did against Barcelona, and Liverpool must cut off Roma's passing at source.

The loss of Strootman means Roma are without a hugely talented passer, but not the most mobile these days after injury problems, and he might have been Liverpool's pressing victim here. Radja Nainggolan should be tucked back into the midfield trio, rather than used in a more advanced role as at Anfield. He's the Roma midfielder happiest playing at a higher tempo, and the man who might start Roma's pressing if they attempt to take on Klopp's Liverpool at their own game.

Compare this midfield with the last Liverpool trio that reached a European Cup final, back in 2007, and it underlines the limitations of the current crop: Javier Mascherano, Xabi Alonso and Steven Gerrard were all genuinely top-class players. Henderson, Milner and Wijnaldum are not. Nevertheless, Liverpool's success in this season's competition owes much to their engine room, and the way Klopp's attackers and defenders help to dominate the Reds' midfield.