Cesc Fabregas was almost at it again last night. The Barcelona midfielder helped set up another goal, this time for Lionel Messi, but it was no more than a consolation as Celtic provided the shock of Champions League Matchday Four with a deserved 2-1 win at Parkhead.

Barcelona are still top of Group G and it would take a minor miracle were they to not stay there. Despite last night’s slip up, their first away defeat in the group stage since October 2006 (2,212 days ago), they remain the team to avoid in the Round of 16 draw.

As much as if not more than Lionel Messi, whose 18 goals in 16 games is a pretty standard return for him these days, Fabregas has been at the heart of Barcelona’s important moments this season. He has scored five goals in his last nine, and this season has created seven for others (more than any other player in Europe’s top five leagues). But more significantly, he has started all ten league games for Barcelona under new coach Tito Vilanova. Against Celtic, he started on the bench and came on with 20 minutes to play.

If Xavi Hernandez represented the type of football Barcelona played under Pep Guardiola, Fabregas could be the player who defines the reign of Vilanova. “I see myself as a replacement for Xavi because my beginnings as a player were in the same position,” Fabregas told Catalan paper El Periodico earlier this week. But it’s not quite that simple.

After a brilliant start to his career at Barcelona last season, Fabregas struggled in the second half of the campaign; partly because he wasn’t fully-fit as he missed pre-season (something Arsenal fans will not remember with fondness) but partly because he was still getting used to his new team-mates and their style of play.

“It’s not enough to have graduated from La Masia to understand the game Barcelona must play,” Fabregas explained. “I moved to England when I was young, and while I was there, Guardiola came up with lots of new concepts that I had not learnt in the training-centre. So I had to catch up.” He was also far from a regular, both in terms of starting matches (23 in the league) and his position: sometimes he was a false nine, other times he tried to replace Xavi or Andres Iniesta.

But most of all, he had to re-educate himself in the Barcelona way and, if anything, try and un-learn bad habits he picked up in England. For example: his coach at Arsenal, Arsene Wenger, always tells his players that their eyes should always face the opposition goal. The Frenchman would let Fabregas go wherever he wanted on the pitch—“I could find my own position”—as long as he was moving forward. “That sense of verticality, it comes from Wenger,” Fabregas explained in an interview with So Foot magazine, published this week.

Fabregas was impossible to mark, because he moved all over the pitch. “I lost the idea of being responsible for my position at Arsenal,” he said, “but at Barcelona, all players are where they are supposed to be.” He described his style as chaotic, a bit like Mesut Oezil at Real Madrid, who has freedom to move anywhere he feels he can create danger.

But that doesn’t work at Barcelona, or at least not in Guardiola’s Barcelona. Their game was more lateral, a game where possession was more important than position. Fabregas has learned to adapt, to respect ‘the philosophy’ of the team, and he knew, despite being captain of Arsenal at 21, and a former captain of Spain Under-21s, that he would have to spend some time on the bench. A few years ago, he could not have managed that, but this is a mature Fabregas, one who can look at the long-term.

And now he is becoming a decisive player for Barcelona. Just as he has adapted to his old, yet new, surroundings, so the team is adjusting to Fabregas. Vilanova’s vision is not the same as that of Guardiola. He knows the team cannot rely on Xavi, who is nearly 33, forever. Last night, Xavi even showed he was human, failing to stop Celtic goalkeeper Fraser Forster’s long kick, an error which allowed teenage striker Tony Watt to score what was the winning goal.

Fabregas may end up replacing Xavi as the midfield talisman, as he said, but he is not a like-for-like replacement. If Xavi represents Barcelona’s lateral style under Guardiola, Fabregas is Vilanova’s vertical equivalent.

Barcelona are a team in transition at the moment; they are switching from the Xavi era to the Fabregas era. It is down to the skill of Vilanova that, with the club top of La Liga and and its Champions League group, hardly anyone had even noticed—until they turned up at Celtic, that is.