LONDON (AP) -On a night of comebacks in the Champions League, the holders left theirs far too late.
Chelsea is facing an uphill battle to qualify for the knockout stage after slumping to a 2-1 loss at Shakhtar Donetsk on Tuesday, the Premier League leaders' first defeat of the season in all competitions.
Manchester United also fell two goals down but staged a rousing fightback to win 3-2 at home to Braga and stay perfect after three group games. Barcelona has the same record after needing a goal deep into stoppage time by Jordi Alba to beat Celtic 2-1.
While those two European giants look assured of progress, Chelsea is battling to avoid becoming the first defending champion to fail to advance from its group.
"We need to win our two home games remaining for sure," Chelsea manager Roberto di Matteo said. "It just shows how the Champions League is a very competitive competition."
Shakhtar moved top of Group E on seven points, three above Chelsea, which was thankful that rival Juventus was surprising held to a 1-1 draw at Danish minnow FC Nordsjaelland. Juve has drawn all three matches to remain unbeaten this season at home and in Europe.
In its 20-year history, no team has retained the Champions League title.
Elsewhere, after its stunning 3-1 win over Bayern Munich last time out, unheralded Belarusian side BATE Borisov came back down to earth with a 3-0 home loss to Valencia to cede its lead of Group F to the Spanish team on goal difference.
Bayern, last season's beaten finalist, is also on six points in a tight group after Thomas Mueller's first-half penalty clinched a 1-0 victory at Lille.
Also, Spartak Moscow beat Benfica 2-1 to get off the mark in Barcelona's group and move a point behind Celtic in the battle for second. Cluj conceded a late equalizer to draw 1-1 at Galatarasary but is still second behind Man United in their group.
Barcelona won its 100th Champions League match and set a new club record of 18 home matches in Europe without a loss, but the win over Celtic was much tighter than expected.
With Lionel Messi in subdued form, Celtic took a shock 18th-minute lead when Javier Mascherano deflected a free kick into his own net and the Scottish side defended stoutly before Andres Iniesta equalized in the 45th for Barcelona.
Celtic goalkeeper Fraser Forster made a string of brilliant saves but was beaten by Alba deep into injury time, the Spain left back glancing in a cross from Adriano.
"It's gut-wrenching," Forster said. "Everybody's devastated."
The competition favorites are top of Group G on nine after three games and victory at Parkhead in the return match in two weeks' time will secure top spot.
United also claimed a third straight win the hard way, after Brazilian forward Alan had taken advantage of some characteristically shoddy defending from United this season to put Braga 2-0 ahead after just 20 minutes.
Javier Hernandez started and completed United's comeback, either side of Jonny Evans' scrappy goal. Alex Ferguson's team is just one win away from qualification, too, which would make up for its surprise exit at the group stage last season.
"It's been the story of our season at home, starting badly and losing goals. We have had to rescue games," Ferguson said, after his team fell behind for the eighth time in 12 games this season.
Chelsea had been undefeated in the Champions League since losing 3-1 to Napoli in a last-16 match in March, but that record was ended thanks to goals by two of Shakhtar's Brazilian contingent.
Alex Teixeira and Fernandinho scored either side of halftime, with Oscar's reply from Chelsea two minutes from time proving only a consolation.
"We didn't play well," Chelsea defender David Luiz said. "It is a shock following some good performances in the Champions League."
Failing to beat Shakhtar in the return match in two weeks' time at Stamford Bridge could leave Chelsea struggling to claim a top-two finish.
The visitors had John Terry returning at center back, wearing the captain's armband embroidered with the slogan "Unite Against Racism" a year to the day since his infamous on-field confrontation with a Premier League opponent that resulted in him receiving a four-match domestic ban for racial abuse.