Strength in depth.

It’s something that we’ve been talking about for years, in some cases not realising it lay right in front of us.

That’s certainly true for Nacho Monreal, a man who joined the club in 2013 for £8million, and it would be fair to say underwhelmed in his early appearances.

He’s not the fastest, but when Arsenal signed him, they believed they were getting a player who reads the game beautifully and makes a fantastic number of interceptions.

In those early months at the Emirates Nacho took some time to adjust to the pace and style of the Premier League, but this season we have seen the former Malaga man really blossom into a quality player.

(Photo by Michael Regan/Getty Images)

Kieran Gibbs has long been heralded as Ashley Cole’s belated heir to the left back position, yet for stretches of the season our Spanish wingback has been keeping his English counterpart out of the team.

Certainly for the tougher clashes, Monreal’s more conservative play has typically been preferred at the expense of some measure of attacking threat, so it was little surprise to see him included in the lineup at Old Trafford.

The beauty of Nacho’s game is it is built on defensive solidity – only when Arsenal have a stable platform to attack does he bomb forward.

He sits close to his centre back and picks his chance to go on the attack. You can see it perfectly evidenced in the build up to his goal, where he is hovering out wide for some time as Alexis and Cazorla toy with United’s midfield before arriving in the box at the perfect moment to deliver the early blow on the way to knocking Man United clean out of the FA Cup.

(Photo by Michael Regan/Getty Images)

He does have a tendency to turn his back on the player and ball when turning to run towards his own goal, but by and large his defensive play is spot on, and Monday night was another illustration of that intelligent play. While he scored the all important first goal, there was also a beautiful moment in the first half which grabbed my attention.

Di Maria (hahahaha) had ghosted in behind and run off the back of Koscielny down our left flank. Perhaps the spell at centre back has helped, but Monreal chased back, and trusting Koscielny to do his job, slotted straight into the space vacated by the Frenchman.

When the cross came in, Nacho was perfectly positioned to clear the danger with the minimum fuss. It was an effortlessly simple movement, but one that showed the intelligence and experience that our number 17 brings to this Arsenal defence.

It would be wrong to gloss over the goal of course! Nacho is so often pigeonholed as a defensively solidly player with little to offer in the opposition half – a fairly damning slight in the modern game where fullbacks are expected to be as adept going towards the opposition goal as to their own – yet his strike against United was a quality finish.

(Photo by OLI SCARFF/AFP/Getty Images)

From my distant vantage point in the very corner of the away end, all I could see was a flash of red boots, and given the position of the player striking the ball and the precision of the finish, I simply assumed it was Alexis.

Imagine my surprise to discover it was Nacho who had applied the final touch with such confidence!

He may not be selected for every game, but he is making a strong case for his inclusion on a regular basis, and that competition for places can only be a good thing.

It’s certainly making for a very watchable battle between him and Gibbs, which can only serve to improve the both of them.

With a game at Wembley to follow this rather satisfying ejection of United from the competition, it’s going to be a fascinating contest.

Over to you, Gibbo…!