Lionel Messi marked his 80th Champions League match with a hat trick on Wednesday as Barcelona began its campaign in Group H with a 4-0 rout of Ajax. Meanwhile, after the euphoric return, reality is biting for Jose Mourinho, and the Chelsea homecoming is quickly going sour, as the team suffered a shock loss to Swiss champion Basel.

Basel had never won a game in England until coming from behind on Wednesday at Stamford Bridge to beat Chelsea 2-1 in Group E.

The winner came in the 82nd minute after Marco Streller seized on slack defense at the near post to meet a corner and flick a header past goalkeeper Petr Cech.

“I am responsible for everything,” Mourinho said, “Especially after a bad result.”

Although Oscar lit up a drab start by curling Chelsea in front before half time, the 2012 European Cup winners and reigning Europa League champions missed a succession of chances to extend their lead after the break.

And Basel, which beat Manchester United in the 2011-12 group stage, made Chelsea pay for its wastefulness when Mohamed Salah began the comeback in the 71st by bending in the equaliser.

Streller then inflicted the loss that came exactly six years after Mourinho’s first three-year Chelsea reign was ended by Abramovich following a drab Champions League draw against Rosenborg.

This was even more embarrassing for the west London club.

Not since October 2003, when Claudio Ranieri was in charge at the start of Abramovich’s ownership, had Chelsea even lost a game in the group stage.

“We go home sad,” said Mourinho, who wants to avoid Chelsea dropping into the Europa League to defend its title.

This setback comes with Chelsea already three points off the Premier League lead after four games, having opened with two wins, a draw and a loss to sit in sixth place.

“We still believe in each other,” Mourinho said. “We need to keep together, stick together and try against Fulham (on Saturday) to get the result to wake up the team for a smile.”

The shock win drew acclaim from Basel’s famous fans.

“Fantastic evening for Basel supporters, wow!” tennis great Roger Federer wrote on Twitter. “I am so happy, thanks FC Basel.”

The win ended Chelsea’s 13-match unbeaten home run in European competitions.

“Winning here at Chelsea against a manager like Mourinho is a great achievement,” Basel coach Murat Yakin said through a translator. “But I’ve spoken with my players and we have to remain modest... there is a lot ahead of us still.”

Basel is only off top spot in Group E because Schalke beat Steaua Bucharest 3-0.

Winning European football elite competition remains the obsession at Stamford Bridge despite the Champions League finally being won under former coach Roberto Di Matteo in 2012.

Against Basel, there was no Fernando Torres in the starting lineup or even on the bench, and Demba Ba only came on in the second half with the teams locked at 1-1.

Instead, Samuel Eto’o, recently signed from Russian club Anzhi Makhachkala, led the attack.

But the Cameroon striker, a four-time African player of the year and the world’s best-paid star while at Anzhi, couldn’t even muster a shot on goal.

“Samuel maybe lacked sharpness and this doesn’t surprise really when you are some years in a place that doesn’t motivate you and you are out of the big stage, maybe you’re there not for the right reasons and you lose hunger and appetite,” Mourinho said. “Now he has that back, he has that motivation.”

Chelsea’s breakthrough instead came from Oscar in the 45th minute, with David Luiz, who had concerned Mourinho with his sloppiness in defence, bringing the ball forward.

The Brazil centre back fed Frank Lampard and Oscar connected with a throughball before striking low past goalkeeper Yann Sommer.

After an insipid first half the type of performance fans jeered during Rafa Benitez’s interim reign last year Chelsea discovered some spark after the break.

But, first the cross bar denied Oscar and, when he tried to curl a shot into the net from a similar position outside of the area, the ball went wide.

The pressure was increasing, but a second goal proved elusive.

A towering header from Branislav Ivanovic was saved by Sommer at close range and the defender saw another header from a free kick nodded clear.

The game really came to life when Basel finally found its stride to stun the hosts.

Slick build-up play allowed Marco Strelle set up Salah to curl in the leveller from the edge of the area.

Then, Chelsea defenders left Streller in space to meet Kay Voser’s corner at the near post and compound the early anxieties for Chelsea at the start of a season that promised so much with the morale-boosting return of Mourinho.

Messi’s hat-trick

The Argentine forward opened the scoring at Camp Nou Stadium with a free kick in the 22nd minute, doubled the hosts’ advantage in the 55th on the break and, after Gerard Pique’s 69-minute goal, added another 15 minutes from time. The goals took his cumulative tally in the competition to 62 nine short of Raul Gonzalez’s all-time record of 71.

Despite some lethargic moments late in the first half, Barcelona eased to the win thanks to Messi’s goals and the team’s near complete control of possession.

And when Ajax did manage to create a scoring opportunity, Valdes was there to keep a clean sheet.

“Barcelona is full of individual talents,” Barcelona coach Gerardo Martino said. “Today we can talk about Messi and Valdes, another day Xavi or Neymar. It is normal that one of them rises to the occasion.”

AC Milan beat Celtic 2-0 in the night’s other Group H game.

True to both teams’ well-established playing styles, Barcelona and Ajax kept the ball on the turf. While Ajax was limited to former Barcelona forward Bojan Krkic’s sprints down the left side, Barcelona soon focused on Neymar cutting in from the left flank.

But it fell to Messi to get the hosts going after he had drawn a foul by dribbling across the edge of the area following a neat passing combination with Dani Alves.

From that prime spot, Messi sent a left-foot shot off the left post, with the ball beating outstretched goalkeeper Kenneth Vermeer before ricocheting into the far side of the net.

Ricardo van Rhijn almost answered for Ajax in the 31st when Bojan picked him out with a cross, only for Valdes to smother his header.

The home crowd turned jittery near the end of the first half, and Barcelona apparently took note by showing the defensive pressure Martino had said was his top priority to restore after the break.

The increased intensity paid off when Barcelona recovered the ball and Sergio Busquets spotted Messi running unmarked down the right side. Messi took his pass, cut back to bring Stefano Denswil to the ground and used his left foot to score.

Neymar, on his European debut, set up Gerard Pique to add Barcelona’s third goal with a long cross.

Ajax lost possession near its area and Xavi Hernandez laid the ball off for Messi to take his time before driving it between a pair of defenders and into the corner of the net for his third goal.

Valdes had his moment of glory one minute later in the 76th when he blocked Kolbeinn Sigthorsson’s penalty kick after Javier Mascherano had fouled Thulani Serero in the box.

Ajax’s loss extended its losing streak to Spanish opponents to nine consecutive games. Ajax’s last victory in Spain dates back even farther to the 1996-97 season when Ajax’s current coach Frank de Boer, who later went on to play for Barcelona, helped the Dutch team win at Atletico Madrid.

The two four-time champions had never met before in Europe’s top-tier competition. Even so, fewer teams from different countries have stronger ties.

Dutch great Johan Cruyff led Ajax to three straight European titles from 1971-73 as a player before moving to Barcelona, where he would later return as a coach to guide it to its first European title in 1992.

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