While all the publicity this week for Lionel Messi’s incredible goalscoring exploits is well deserved, it’s also worth noting just how superbly Barcelona have defended recently. The Argentine’s hat-trick against Viktoria Plzeň in Tuesday’s Champions League group game brought him to 202 goals for the Catalans, but the game was also memorable for Victor Valdés breaking Miguel Reina’s club record for the longest ever run without conceding a goal. Since Pablo Hernández scored in the 2-2 draw with Valencia on September 21st, Barca have kept nine successive clean sheets. The run is now at 877 minutes, and counting.

This is all the more remarkable considering how unsettled Barcelona’s defensive line-up has been lately. Tuesday was the first time Pep Guardiola’s supposed first choice back four - Dani Alves, Gerard Piqué, Carles Puyol and Éric Abidal - had played together this season. Piqué and Puyol, the team’s two recognised centre-backs, have both been plagued by injury, and even as they return to fitness Guardiola has been very wary about playing them. He’s been able to be so careful as his side’s outstanding defender this season has been ex-Liverpool midfielder Javier Mascherano.

This time twelve months ago not many Barca or Liverpool fans would have predicted that last sentence. Mascherano did not settle quickly after his acrimonious move from Anfield in summer 2010. He was lucky not to be sent off before being substituted at half-time on his debut in the shock 2-0 defeat to Hercules. He did not start another La Liga game until October, amid claims he couldn’t move the ball quickly or cleverly enough to fit within Barca’s tiki-taka style. Notably, he was an unused substitute in Barca’s biggest pre-Christmas game - the 5-0 win over Real Madrid at the Camp Nou.

After the turn of the year though Mascherano began to play more often. With Gabriel Milito and Andreu Fontás at either ends of their careers and Puyol and Abidal missing through injury and illness, Guardiola moved Sergio Busquets to centre-half and Mascherano played more regularly in his place. That plan though was hampered by Busquets’ lack of pace, and it still left the Argentine clogging up the centre.

Then, for the first leg of the Champions League quarter-final against Shakhtar in April, Guardiola swapped them, starting Mascherano at the back for the first time. Pep explained the unexpected decision after his side won the game 5-1  - “I wanted his speed at the back, and also his experience.” It still seemed odd, particularly given Mascherano is only 5’7” tall, but it worked.

Puyol was fit for the 1-1 league draw with Madrid, but when he got injured again, Mascherano was back in and started 13 of Barca’s last 15 games, coming on as sub in the other two. He played centre-half in both semi-finals against Real Madrid and in the final win over Manchester United, three games in which Barca only conceded two goals against their major Spanish and European rivals.

Guardiola again decided against signing any defenders last summer, instead letting Milito leave and stocking up further forward with Cesc Fábregas and Alexis Sanchéz. This was not because Fontás was ready to step in – the young defender has only played 79 minutes all season, in a 2-2 draw at Real Sociedad. It was also not (or not just) because Pep dreams of a team of midfielders who use possession as the best form of defence. Barca didn’t need another centre-half because Mascherano is now seen primarily as a defender. In his 13 games this season the Argentine has not once played in midfield. His performances at the back have been so good that Guardiola told reporters last week that “he is the best centre-half we have at the moment.”

This is not to say that the transition to defender has always gone absolutely perfectly. Mascherano was exposed by Alexandre Pato in the 2-2 home draw with Milan in this year’s Champions League opener and he had a shocker on the right side of a back three (with Alves further up the pitch) in the first half of the 2-2 draw at Valencia. But Guardiola reacted at half-time in that game by moving Alves back and Mascherano across and Barca have not conceded since.

El ‘Jefecito’ (the little chief) has also emerged as Barca’s on-field defensive organiser. Whenever Guardiola makes a tactical changes during games - which is often - Mascherano usually gets called over to the sideline for instructions which he then passes on to his team-mates. He is also reportedly a leader in the dressing room, and someone who even Messi turns to for advice.

The player himself seems understandably happy with his new role:

“The boss seems to prefer me at centre half, rather than in the middle,” he told Barca TV the other week. “It’s not my natural position and I know I have a few problems, but I just try and do my best. I’m not that tall, but I try and overcome that by trying to combine commitment and concentration. Opportunities are there to be grasped and taken advantage of.”

It’s fair to point out that Mascherano’s opportunity only arose due to injuries to Barca’s first choice defenders. It’s also likely the case that those who said he couldn’t play Busquets’ ‘pivote’ role were right. But it’s true that Mascherano has developed into a top class defender, suited to Barca’s unique playing system, unbelievably quickly. Valdés has made some decent saves, but Barca’s record breaking non-conceding run is mainly due to their new key defender’s intelligence and adaptability, and his coach’s ability to spot it.