If Borussia Dortmund's struggle to overcome Malaga in the UEFA Champions League quarter-final told us anything, it's that Mats Hummels is a sorely missed presence.
Manuel Pellegrini's side set up roadblock after roadblock whilst simultaneously exposing BVB's weaknesses, and the presence of the German international would have made things much easier for the favourites.
The two-legged affair served as a reminder to us of why he is so important to Juergen Klopp's side, so let's recap.
In both legs against Malaga, Dortmund really struggled with the high ball.
The full-backs, Marcel Schmelzer and Lukasz Piszczek, are not aerially confident and seem to look for a leader to really take control of the situation.
Felipe Santana is an admirable stand-in, but BVB's progress in the quarter-final was always under threat when Willy Caballero lofted a long ball into the path of Julio Baptista.
Baptista would either win the header or cause chaos in the process, and it was his good work that allowed Joaquin the chance to open the scoring at Signal Iduna Park.
From then, induced by fear, Dortmund's centre-backs instinctively dropped five yards when the ball was lofted toward Julio, giving him time and space to bring it down if he wanted to.
Mats has the presence, the physicality and the ability to stop this becoming a regular issue, and if Malaga had looked to their target man more often, they could have had more joy.
But it's not just in an aerial sense that Hummels improves the side.
His tackling, timing and marking are all of the highest quality, and as B/R's Bundesliga expert Clark Whitney likes to say: "Mats marks opposing strikers into absolute anonymity."
Some of the best forwards in the world have come up against him and failed to get a kick. Be it Bayern's ferocious forward line, Real Madrid's deadly duo or Ajax's free-flowing fiesta, Hummels pockets everything.
Klopp has good backup in Santana and a great partner for the majestic Mats in Neven Subotic, but the Serbian is more of a hard-hitter, and there are, at best, two players in the world that could fill Hummels shoes.
And perhaps the reason for that is his attacking prowess.
He doesn't just mark you off the pitch in a defensive capacity, when he gets the ball he drives forward and initiates attacks. He brings midfield markers and strikers to him, forcing them to close him down, and that opens things up in the middle of the pitch.
Malaga looked near impregnable for 130 minutes, and a big part of that was their organisation and discipline at the back. Jeremy Toulalan and Ignacio Camacho formed a wonderful holding duo, and Martin Demichelis had the game of his life (again).
Duda was fielded in an advanced role to close down Ilkay Gundogan and give him as little room to work with as possible, while Toulalan pocketed Mario Goetze.
It was looking borderline impossible for die Borussen to break los Boquerones down, and it came down to a genius flick from Marco Reus to get the ball rolling.
With Gundogan shut down and the midfield battle lost, Hummels was the man who could have really changed proceedings. Surging forward, Duda and Baptista would have had no choice but to close him down.
When teams bottleneck against Dortmund and account for every attacking player, Mats is the one who can break the system down, much like Gerard Pique does for Barcelona (although they're not comparable in any other sense).
Heading into the semi-final, Hummels cannot be missing else BVB stand a serious risk of falling at the final hurdle.
Statistics, graphs via CNN Football Club.