In the 79th minute of Marseille's match at Rennes on Friday, the visiting team's 19-year-old forward, Jordan Ayew, broke down in tears. Was he injured? Did someone call him a nasty name? No, he had just teed up the goal that put his team 2-0 up against the team who were second in Ligue 1 on goal difference and he was crying with relief.

He had wasted a glaring chance earlier in the game, as he has done on several occasions this season during his intermittent appearances from the bench, so he was thankful he had been able to atone and play an important part in a victory that keeps alive Marseille's hopes of retaining the title they won last season for the first time since Ayew's father, Abedi Pelé, was one of the team's stars.

Ayew's redemption came amid a collective show of defiance from Marseille that will have alarmed at least one Scottish spectator in the Stade de la Route de Lorient – Sir Alex Ferguson, whose side must beat the French champions at Old Trafford on Tuesday to reach the Champions League quarter-finals.

Ferguson may have noted that rumours of Marseille's demise seem to have been exaggerated. The Rennes game had been identified by many as the one in which their faltering season would definitively unravel. They went into it under intense pressure. Not only had they been outplayed and beaten at home the previous week by the league leaders, Lille, but the camp was then beset by scandal: on Tuesday, Brandão, the striker who lined up against United in the first leg, was arrested on suspicion of rape. After being held in custody for two nights, Brandão was allowed to return home to Brazil pending further action and the club have said he may not play for them again.

The allegations against Brandão, and the media frenzy that followed, exacerbated an already tense atmosphere. Rennes, who went into Friday's game on the back of a five-match winning streak, were tipped to take advantage. Instead, Marseille twice pierced the best defence in Ligue 1 during a performance that convinced their manager that, although the team remains imperfect, they truly have the stuff of champions.

"Many people said we would collapse, but we're still here, with our qualities and our flaws," said a beaming Didier Deschamps (right). It seems that, instead of taking on a team in crisis, United will meet a side undergoing a renaissance.

Or perhaps not. It really is difficult to know with this Marseille team. The most obvious of the flaws Deschamps mentioned were the lack of cohesion and the sloppy passing his team displayed in the first 20 minutes at Rennes, a recurring theme of the campaign. However, once Loïc Rémy put them in front, Marseille flaunted their qualities.

André Ayew, Jordan's older brother and one of the few consistent Marseille performers this season, dazzled on the wing; Lucho González, the Argentinian playmaker who shone last season but has struggled this term, began to find his passing range; and Deschamps's tactical rejigging in response to recent poor form worked. The manager moved Gabriel Heinze from left-back, where his lack of pace was being exposed, to the centre of defence, reintroducing Taye Taiwo on the left – and the Nigerian's enterprise going forward lends an extra dimension to Marseille's attack.

The other consequence of Heinze's relocation was that the erstwhile centre-back, Stéphane Mbia, was redeployed as a holding midfielder. Marseille had been overrun in midfield too frequently recently; Mbia seems to have solved the problem.

But perhaps the most significant development at Rennes was that Mathieu Valbuena returned to action without a hitch. The attacking midfielder featured only in the last seven minutes, but suffered no adverse reaction to the knee injury that had afflicted him for the previous six weeks, so he should start on Tuesday.

Valbuena is only 5ft 4in and, as a teenager, was released by Bordeaux on the grounds that he was too small to make it as a professional. But after making peace with Deschamps, who had initially made it clear the pet of the previous manager had no place in his plans, Valbuena became a key part of Marseille's title-winning team and one of the few champions to excel consistently this term, until his injury. He provides most of the artistry to a team built primarily to be solid. "We need to be daring in Manchester, you can't achieve anything if you don't show adventure," he declared ahead of Tuesday's match.

Deschamps is unlikely to be so bold, but it may not be a shock if Valbuena has the last word. He does, after all, have previous in England: he scored a sumptuous winner for Marseille against Liverpool at Anfield in 2007, and, last November, was the choreographer as France led Fabio Capello's stiffs a merry dance at Wembley.

Only fit enough to appear fleetingly as a substitute against United in the first leg, Valbuena islikely to start wide on the right at Old Trafford, and work his way infield, all mesmeric dribbles and cute passes. He may be small, but he tends to stand out.