What we learned from the Champions League: Llorente actually made Spurs worse...

Champions League knockout ties are meant to be tight -- instead, we were treated to 18 goals in the four quarterfinal matches this week, with almost non-stop action from Lionel Messi's brilliant opener for Barcelona against Manchester United to Raheem Sterling's disallowed goal against Tottenham in the final minute. Here are some key talking points from a wonderful week of European action.

Manchester City 4-3 Tottenham Hotspur (4-4 on aggregate): Llorente actually made Spurs worse...

At the end of the most bewilderingly dramatic, end-to-end Champions League knockout match you'll ever witness, it was difficult to analyse the game rationally. Manchester City won 4-3 on the night but lost overall on away goals, having briefly thought they'd snatched a last-gasp winner through Raheem Sterling before VAR ruled it out for offside.

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However, the silliest thing about the whole match was the fact that the man who scored Tottenham's "winner," substitute Fernando Llorente, shouldn't have been introduced by Mauricio Pochettino in the first place.

He may have performed his job by scoring the crucial goal, yet you can't give him too much praise for converting a corner via his forearm (though VAR ruled it fair on review), then his hip and into the net. In fact, the introduction of the Spanish striker caused Spurs serious problems. Until then, they'd looked mobile enough to battle against Manchester City in midfield, and offered a consistent counter-attacking threat through Son Heung-Min and Lucas Moura that saw them score twice inside 10 minutes.

But, as City hit back to take a 3-2 lead that still saw Spurs ahead on aggregate, the 41st minute departure of the injured Moussa Sissoko represented a serious problem as Pochettino didn't have a like-for-like replacement.

The introduction of Llorente with over half the match remaining was peculiar: it robbed Tottenham of their counter-attacking threat and meant they were forced to withstand serious pressure without offering much going the other way.

Pochettino could have brought on left-back Ben Davies and moved Danny Rose forward, introduced defender Davinson Sanchez and moved to a system with three centre-backs, or shown faith in young midfielder Oliver Skipp. Any of those options would have given them ammunition for their main weapon in this game: speed on the break.

In the end, Tottenham progressed, Llorente was the hero, and few fans will be too worried about losing the tactical battle. But it only served to showcase how limited Pochettino's options off the bench were.

Porto 1-4 Liverpool (1-6 on aggregate): Henderson has to play against Barcelona

Jordan Henderson shouldn't be too concerned by his omission from Liverpool's starting XI for the victory in Portugal for three reasons.

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First, Jurgen Klopp was clearly rotating his starting XI, also omitting Roberto Firmino and in-form Naby Keita. Second, Liverpool struggled to control the midfield in his absence. Third, it took Henderson just eight minutes to provide a decisive contribution, whipping in a tremendous cross to allow fellow substitute Firmino to nod home. Had the Brazilian not got there first, Sadio Mane probably would have converted at the far post.

It was the fourth game in a row where Henderson had provided a crucial contribution: a goal and an assist as a substitute at Southampton; a lovely disguised pass for Trent Alexander-Arnold to cross for Firmino in the first-leg win over Porto; a lovely stabbed cross for Mane's headed opener against Chelsea; and now this assist.

Henderson will presumably start for the trip to Cardiff this weekend -- the type of match Liverpool's all-action skipper should be well suited to. More significantly, it's now difficult to imagine him being omitted from the semifinal against Barcelona, where his tenacity and energy should be crucial in ensuring Liverpool enforce their own style on the game, rather than merely responding to Barca's possession play.

Barcelona 3-0 Manchester United (4-0 on aggregate): Manchester United have a decision to make on David De Gea

After a 1-0 defeat in the first leg, Manchester United were always facing an uphill task in Barcelona this week and, already 1-0 down courtesy of Messi's stunning 16th minute opener, David De Gea's horrendous error to let Messi's shot slip though him for the second goal virtually ended the tie minutes later.

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De Gea has endured his worst season at Manchester United, with the arguable exception of his first campaign in 2011-12 when he struggled to adjust to the physicality of English football. Although still capable of producing man-of-the-match displays, as in United's 1-0 victory at Spurs early this season, recently too many errors have crept into his game for a goalkeeper with his reputation.

De Gea has only a year left on his contract at United, and sources have told ESPN FC that the club will refuse to sell this summer, hoping that he signs a new deal but risking losing him on a free. Yet they must also be giving serious consideration to cashing in, considering his dip in form.

The question is where De Gea would go? Real Madrid now have Thibaut Courtois, even if the Belgian hasn't impressed this season; Barcelona and Atletico Madrid are happy with Marc-Andre ter Stegen and Jan Oblak, respectively, and the fact the latter has just signed a new contract makes him even more difficult to sign.

The other issue is who United would find as a replacement? As Chelsea demonstrated last summer when spending a world record £71.6m on the competent rather than outstanding Kepa Arrizabalaga, sometimes there simply aren't any adequate goalkeepers worthy of breaking the bank for.

United might end up giving Sergio Romero, a dependable deputy and World Cup finalist with Argentina, a season between the posts as they await a serious replacement. The club have a variety of serious decisions to make this summer, particularly in the defensive section of the side, but the biggest involves their most consistent player over the last half-decade. It might be time for him to move on.

Juventus 1-2 Ajax (2-3 on aggregate): Ajax's switching of wingers key to their success

There's an understandable determination to relate Ajax's success back to the glory days of the 1970s and the 1990s, and there are clearly some common factors: the insistence upon possession football, the commitment to developing young talent.

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But there are also a couple of things Ajax are doing differently. For a start, their squad-building approach has shifted towards spending serious money on established players from major leagues: Daley Blind and Dusan Tadic are not the type of players Ajax would have bought five years ago, for example.

On the pitch, Ajax don't play with outright wingers like the sides of the 1970s and the 1990s. And it's not merely that these players are fielded as inverted wingers, checking inside onto their stronger foot and shooting -- as is now the standard practice across Europe. It's that Ajax's wingers relentlessly cross the field during attacks, switching to the opposite channel and combining directly with their teammate on the opposite side.

Most commonly, this means David Neres moves from the left flank to an inside-right position, doubling up with Hakim Ziyech to overload the opposition left-back. Typically, Ajax work a passing combination with several players on that side of the pitch, before looking to switch the play to an unmarked overlapping left-back -- although this proved more difficult against Juventus without the suspended Nicolas Tagliafico.

Ajax have found the right balance between respecting their traditions and adjusting to the modern game. Having outplayed both Real Madrid and Juventus away from home, there's no reason they can't do the same to Tottenham.