Diego Costa remains a major doubt for Atletico Madrid’s Champions League quarterfinal first leg against Barcelona at the Camp Nou, although reports suggest he is set to be named in the starting lineup. The forward has scored 33 goals this season, but left training early on Monday with a groin injury, after which manager Diego Simeone claimed that it was “unlikely” that he would feature in what is arguably Atletico’s biggest match this century. Speaking before Costa’s exit from the Camp Nou pitch, Simeone was happy to admit that Costa was every bit as important to Atletico as Lionel Messi is to their opponents.

“Without doubt,” he responded, according to Spanish sports daily AS. “Cristiano [Ronaldo], Messi and Costa, all of their own style, but are all equally important. Two in form players now face off, and I think that Diego is growing day by day, and Messi is demonstrating what good those two months off did him, and will go into the World Cup in top form.”

Latest reports in Spain suggest that Costa will ultimately make the starting lineup, although whether that means he is fully fit remains to be seen. It would also still leave a question of where exactly he will play. Simeone has had success against Barcelona previously this season by lining up Costa out wide and utilizing him as a potent weapon on the counter attack. Indeed, all three matches between the sides this season have so far ended in stalemate. Simeone, though, is expecting Tuesday’s encounter to be the most difficult of the lot.

“I think tomorrow's match is the toughest of all our meetings with Barcelona,” he explained, according to Uefa.com. “Barca are back to their best and tomorrow we will see that version of them. They will want to decide the tie tomorrow, not in the second leg. We will have to make the most of our chances. If we say we're favorites, we're cocky. I just say we're here to compete. This will be a great tie and tomorrow's match is very important.”

Simeone has cause to be cautious, with Barcelona having arguably played their best soccer under Gerardo “Tata” Martino in the past few weeks. An impressive win over Manchester City in the last-16 of the Champions League was followed up by a thrilling 4-3 win over Real Madrid at the Bernabeu that got them right back into the title race. Barcelona now trail leaders Atletico by just a single point in La Liga.

Another factor counting in Barcelona’s favor is the form of Messi. Not fully fit in any of the three previous matches between the sides this season, the Argentine is now looking fully up to speed after getting a much-needed rest as a result of a hamstring tear. Messi has scored nine goals in his last five matches.

While Messi’s place as the fulcrum of Barcelona’s attack is secured, much conjecture surrounds who will start alongside him. In the big games of late, Martino has started Andres Iniesta on the left to help provide a greater presence in midfield. That has meant Neymar, who, in contrast to Messi, has struggled since returning from injury, being pushed over to an unfamiliar right-sided role.

“When a coach has an idea, he needs to field the players in the right positions for it,” Martino said. “We know we sacrifice Neymar on the right side. We analyze such things after the match rather than before, but players of Neymar's caliber can play anywhere in attack.”

Neymar could even be out of the side entirely, with Pedro having returned to training after illness. Barcelona will be without two of their most experienced campaigners, however, with Victor Valdes ruled out for the long term after suffering an anterior cruciate ligament injury, while Carles Puyol has still not been cleared to play.

While sweating on the fitness of Costa, Simeone will definitely be without midfielder Raul Garcia, through suspension, and second-choice right-back Javier Manquillo because of injury.

Where to watch: The first leg of the Champions League quarterfinal will kick off at 2.45 p.m. ET. Coverage will be provided by Fox Sports 2, with a live stream available via Fox Soccer 2Go.