Diego Costa drew another blank last weekend, which was beyond frustrating for him. Atletico Madrid were playing Sevilla at the Ramon Sanchez-Pizjuan Stadium—a club he has scored 11 times against, more than against any other team in La Liga—but he was left on the bench for kick-off.

It was the second time in three league games that Costa lost his starting place. This is new territory for him—previously Costa only sat out games for Atletico because of injury or suspension, but he is no longer a guaranteed starter.

When his coach, Diego Simeone, summoned him for the second half, Costa was in the thick of the action, but again he failed to register a goal. He had a strike disallowed for offside and missed a penalty, which cost Atletico Madrid two points—as the game finished 1-1—and the outright leadership at the top of the La Liga table.

Costa has only scored two goals—one a penalty—in 14 league and UEFA Champions League games this season. When given a starting berth for Atletico's UEFA Champions League game Wednesday against Bayer Leverkusen, he again failed to score during the 2-1 defeat and was withdrawn on the hour mark. His slump in form is following a pattern. Last season, he scored two goals in 16 league games for Atletico; and the season before, three goals in 15 league games. Across all competitions, he has scored just 14 goals since returning to Madrid.

The Brazilian-born striker hasn't featured for the Spain national team since Russia's FIFA World Cup in the summer of 2018, though he did withdraw from the squad's UEFA Nations League games in September 2018 for personal reasons. Inako Diaz-Guerra, a journalist with El Mundo, believes the problem could be in his head.

"The reality is that the Diego Costa who returned to Atletico doesn't look anything like the guy who left in 2014, the guy who was the team's top goalscorer the season they won the league," says Diaz-Guerra.

"During pre-season his form was very good. He was in good physical shape. He scored four goals against Real Madrid, but since the season started he's played poorly. He hasn't been fighting. He's suffered from a lack of confidence. I think the problem is more psychological than physical.

"Costa is a player who has to be finely tuned. He has to be up for it, pressing defenders, getting involved in pitch battles, but now if you look at him, he's cold. It's not something you associate with him on the pitch. The lack of goals has affected his confidence. He looks like a man who is disconnected."

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The worrying aspect for Simeone is that there are no mitigating circumstances. "Now it's difficult to see the old Costa returning," adds Diaz-Guerra. "Over the last couple of years, there were other factors. He suffered from injuries. He had the six-month absence because of the transfer ban. But now physically he's in good nick—he's not injured; he has his ideal fighting weight. There's no excuse for him not to perform, but he's not the player he was."

Costa re-joined Atletico Madrid in the summer of 2017 after a messy exit from Chelsea. At the time, he was Atletico's record signing, arriving for a reported fee of €65 million. He missed the first half of the season, though, because FIFA banned Atletico from registering players until January 2018. It looks as if the half-year absence from the game cost the striker.

"Costa's dip in form can be traced back to the six months he sat out during Atletico's transfer ban," says Euan McTear, author of Hijacking La Liga: How Atletico Madrid Broke Barcelona and Real Madrid's Duopoly on Spanish Football. "He left Chelsea at the start of the 2017-18 season because he fell out with [then Chelsea manager] Antonio Conte. He arrived at Atletico, but he had to wait until January to play. When he came back he was his lively self during the first few games but since then, he's had the dip.

"That time being inactive must have played a part. It makes sense—when you're an older player, you don't want to lose your touch or the knack of things. Think also about other players who suffered that FIFA transfer ban—Aleix Vidal and Arda Turan at Barcelona. Both players were signed by Barca in 2015 because they were good players. What happened to them after that six months out? We never heard from them again."

Costa played with Chelsea for three seasons. He was a gold-standard striker, netting 20 goals in both the club's championship-winning seasons, 2014-15 and 2016-17, and he's still in the top 10 fastest strikers to score 50 goals in English Premier League history.

Costa won two Premier League titles at Chelsea Clive Rose/Getty Images

It's this striker—and the one who scored 27 goals during Atletico's title-winning campaign in 2013-14—that Simeone is trying to find again. It's why Simeone has persevered with him—when he has lost patience quickly with reams of other strikers, from Jackson Martinez to Kevin Gameiro, over the last eight years—despite Costa's diminishing returns.

"Simeone knows that if Costa is at his best, he is the ideal forward for Atleti's playing style," says Fran Guillen, author of Diego Costa: The Art of War. "Diego is a man capable of stretching the opposition team by himself and fighting a whole defence with no one to help him. For a team that usually defends deep, someone like that in attack is perfect. When Costa is fit, he is a spectacular player. He is an army by himself, like a stampede of buffalo."

It's not that Costa has been unprofessional off the pitch. He came back looking lean for pre-season training in the summer.

"When Costa causes all these rows on the pitch, it gives the impression he's a troublemaker, but Costa at Atletico has never caused any problems off the pitch," says Diaz-Guerra. "He's professional in his training. He's very popular in the dressing room with his team-mates. All the new, young players who joined this season—Renan Lodi and Joao Felix, for example—have all commented about how he has helped them to fit in.

"Costa is a big child. He's naughty. He likes fooling around, carrying out pranks, making uncouth videos. He's rough, but in reality he never causes serious problems like some footballers, like partying late at night or causing controversies in the press with comments. He's not a Neymar."

Atletico's fans can be as grumpy as the next. They used to whistle Antoine Griezmann when the Frenchman played with the club. Only a couple of weeks ago, they got on the back of club captain Koke, who has been at the club for two decades, whistling him when he was substituted against Bayer Leverkusen in the 1-0 UEFA Champions League win. But Atletico's fans remain steadfast in their support of Costa. He's as popular with them as he is with his Atletico team-mates.

"Atletico's fans adore him," says Patricia Cazon, a journalist with Diario AS. "They consider him one of their own. He's la furia (the fury, the rage). He's a warrior. They love him. It's why they have kept patience with him."

But their patience can't be endless. Costa turned 31 last month. Diaz-Guerra points to a natural decline after such a long grind. Costa has miles on the clock—he's been toiling as a professional footballer in Europe since being transferred to Braga almost 14 years ago.

The big performances—like his two-goal masterclass in Tallinn, Estonia, against Real Madrid to help Atletico win the 2018 UEFA Super Cup final; or his bullying of Leonardo Bonucci and Giorgio Chiellini when Atletico turned over Juventus 2-0 in last year's UEFA Champions League round-of-16 first leg at the Wanda Metropolitano—have become rarer and rarer.

Alvaro Morata, who scored Atletico's consolation goal midweek against Beyer Leverkusen, his fourth goal in four games, is becoming the team's reference point up front.

"Each passing year leaves us witnessing less afternoons of Costa at his best level," says Guillen. "Morata is performing reasonably well, and it is an open secret that Atletico plans to sign a forward in the winter market. The team is registering its worst scoring stats in its history, with only 12 goals in its first 12 games of the league campaign. Also the talk about offers for Costa from China and Brazil are becoming more common."

Time is running out for Costa. After a lifetime of butting heads with the finest and most fearsome defenders of the modern age in football, Costa's struggle to resurrect his Atletico career looks as if it's his toughest fight yet.

Follow Richard on Twitter: @Richard_Fitz