Barcelona's final goal in last night's 4-0 Champions League defeat of Milan was a typical Jordi Alba strike – breaking forward at speed down the left, finishing with the predatory instincts of a centre-forward. It was reminiscent of his Euro 2012 final goal – where he demonstrated his ability to inject pace and directness into an otherwise conservative Spanish passing side – and wasn't dissimilar to his consolation in the 3-1 Copa del Rey defeat to Real Madrid two weeks ago. But, crucially, it was completely out of keeping with Alba's performance. His stamina is unquestionable, and his attacking threat throughout 2012-13 has been considerably greater than that of Daniel Alves on the opposite flank – but Alba's display against Milan was, by design, his most cautious since joining the club last summer.

Rather than replicating the system used in the first leg, where Milan got tight to Lionel Messi, Jordi Roura (perhaps with help from Tito Vilanova on the day it was announced he is set to return from cancer treatment in New York to Barcelona before the end of the month) used a different formation.

Everyone wanted David Villa recalled to play his traditional role – starting wide, before cutting inside into the space Messi creates by dragging the centre-backs up the pitch. Villa was recalled, but Barcelona did something different. They used Villa as a centre-forward, pressed high up against Philippe Mexes – the man who had shut down Messi so effectively in the first leg. In the game following the 2-0 defeat in Milan, a 2-1 win over Sevilla, Villa was introduced to play as a centre-forward.

"With Villa upfront in the second half, Sevilla's centre-backs couldn't move forward, which gave me more space," said Messi.

Roura replicated that system. As Villa occupied Mexes, Messi found space just behind. Villa was there as a blocker, a decoy, a role designed to bring the best out of Messi – strange, as they have not always combined effectively since Villa joined in 2010. "We fixed our centre-forward position," said Roura. "That prevented their defenders from coming forward."

Villa's individual goal threat was almost incidental, although he finished excellently for the third, his celebration demonstrating how much it meant after a difficult 18 months.

But by Barcelona using Villa centrally, they needed an extra player to stretch the game on both sides, preventing Milan from staying narrow. So Alves was pushed forward to play – even by his standards – an advanced role on the right. Pedro Rodriguez played as winger on the left. With Messi cast as a No10 ahead of a midfield diamond, Barcelona were not too far from the 3-4-3 system Pep Guardiola tried to introduce last season, with mixed results.

For Alba, that meant a completely different role. It wasn't always a back three – when Milan had continuous spells of possession, Alves dropped back into position and Barcelona reverted to a back four. But the nature of the game meant this was extremely rare – Barcelona inevitably dominated possession, with Sergio Busquets and Javier Mascherano stepping forward to regain possession immediately – and Milan concentrated on attacking directly through Stephan El Shaaraway and M'Baye Niang. Alves was located high up the pitch, Alba tucked inside.

Uefa's average position diagrams tell the story – in the 15 minutes before half-time, Alves was level with Pedro, while Alba tucked inside in the same role Gerard Piqué was playing. "We had a very clear idea of how we needed to play, we tried to open up the pitch on one side more than the other," said Roura. While El Shaaraway caused problems and Niang had Milan's best opportunity of the night, Alba kept Kevin-Prince Boateng quiet.

Like many modern full-backs, Alba was originally a winger – as recently as last season, he started 10 games on the left wing in Valencia's 4-2-3-1. Even when he played at full-back, Unai Emery used the tall and powerful Jérémy Mathieu ahead of him to help with defending – the Frenchman is a left-back by trade, and played at centre-back in last week's defeat against Paris Saint-Germain. Part of the reason Alba was so attractive to Barcelona – as well as his La Masia schooling and his understanding with future team-mates forged in the national team – was that every time Valencia played Barça, Mathieu and Alba would run riot down Valencia's left flank.

"Emery had a lot to do with the decision [to use me at left-back]," he said upon signing for Barça. "It gave me the chance to go to the European Championship, and to come back to Barça as a full-back or winger. It's changed me a lot: a year ago, I'd never imagined I'd go to the Euros as a full-back."

Last night, he was partly a full-back, partly a centre-back. Barcelona have made a habit of converting midfielders into centre-backs – Yaya Touré played there in the 2009 Champions League final, Sergio Busquets has dropped in occasionally and Javier Mascherano has successfully made the switch, convincing Barça to try the same with Alex Song. No other side in Europe would willingly use a 5ft 7in former left-winger as part of a back three.

If there was any doubt about Alba's defensive qualities, he answered the critics with a superb last-ditch challenge to deny Robinho in the 82nd minute, when Barça were a goal away from being eliminated. It was the most crucial defensive incident in the match, so impeccably timed, that it was half-tackle, half-block. Alba's challenge was as important as his goal and summed up his strict defensive role.

Of course, even when used as a pure defender, Alba can still attack. Three of his last four Barcelona goals have come in the 94th, the 89th and now the 92nd minute. Last night, it wasn't because Alba didn't stop running – it was because he knew when to start running.

Michael Cox is the editor of tactics website zonalmarking.net