Celtic are still without a point in Europa League Group G as Bayer Leverkusen produced an impressive display to come away with a 4-0 victory at Parkhead on Thursday night
The German side, who sit second in the Bundesliga, raced into a two-goal lead at the break as Piero Hincapie's opener (25) following a slip by David Turnbull was added to by the exciting teenage prospect Florian Wirtz 10 minutes later as the visitors pounced on another mistake from Anthony Ralston
Kyogo Furuhashi had seen two presentable opportunities pass him by with the game level as Lukas Hradecky kept out his effort while a piece of last-ditch defending from Jonathan Tah prevented a certain goal inside the opening minutes, but Leverkusen's superiority and class eventually told
Referee Marco Di Bello awarded them a second-half penalty when he adjudged Cameron Carter-Vickers to have handled from Paulinho's shot with Lucas Alario tucking home the ensuing spot-kick (58) and there was still time for substitute Amine Adli to complete a routine victory with an emphatic finish in stoppage time (90+4)
The result means Celtic already find themselves in a difficult position to reach the Europa League knockout stages after two defeats in Group G, as Postecoglou turns his focus towards a huge Scottish Premiership encounter away to Aberdeen on Sunday, live on 
Postecoglou's side have now won only two of their last eight games in all competitions, including a 4-3 defeat by Real Betis in their Group G match
Celtic are in sixth place in the cinch Premiership, six points behind leaders Rangers and return to league duty at Aberdeen on Sunday and will be boosted by aspects of their performance and by having some key players back
There was a lot of speculation around which of the Celtic players recovering from injury would start the game
Hoops fans in a packed and noisy Parkhead were delighted to see the return of captain Callum McGregor and striker Furuhashi with Greek striker Giorgos Giakoumakis, yet to make his debut, on the bench
Former Celtic right-back Jeremie Frimpong was in the Bundesliga side but striker Patrik Schick, who scored a double - including a wonder goal - for Czech Republic at Hampden Park in their opening Euro 2020 game, was among the substitutes
The game burst into action
 Within a couple of minutes, Leverkusen striker Alario had the ball was in the back of the Celtic net after keeper Joe Hart had spilled a shot from Wirtz, only to be ruled offside
Seconds later at the other end, Furuhashi sped into the box and rounded keeper Lukas Hradecky and set himself to stroke the ball into the empty goal but was robbed at the last second by defender Jonathan Tah before Hradecky made saves from Furuhashi and defender Carl Starfelt
Hart had to be smart a close-range effort from Moussa Diaby but he had little chance when the German side took the lead
Turnbull slipped near the byline which allowed Mitchell Bakker to cut the ball back for Hincapie to sweep the ball in from 16 yards despite the efforts of young left-back Adam Montgomery, also back in the side
The Celtic defence was in disarray when Paulinho played in Wirtz who knocked the ball past Hart and into the net which left a very different complexion on matters
Furuhashi tested Hradecky again at the start of the second half with a shot on the turn before the keeper made an even better save from Jota with winger Liel Abada missing the target with the follow-up shot
A penalty shout when Frimpong knocked over Turnbull was ignored by referee Marco Di Bello but when the former Hoops defender drew a save from Hart with a drive, Paulinho's shot from the rebound was judged to have hit the arm of defender Cameron Carter-Vickers
Alario sent Hart the wrong way with the spot-kick and the tie was effectively over
Moments later, Hradecky brilliantly saved a header from Jota and Furuhashi hit the side netting with the rebound and another great chance was missed
In the 74th minute, amid a glut of substitutions, Giakoumakis replaced Turnbull but it was Hart who had to make the next save, a header from Schick, who had also come on before Adil hammered in a fourth on the break
 Now Celtic must regroup and get ready for the Dons
Celtic boss  told  "It's disappointing as we had some great chances
 It's the story of both boxes but they are a good side
 They were clinical inside the box and we were wasteful
 We made some mistakes that you can't make at this level
"So it's disappointing overall
 There were a couple of moments where Kyogo may have felt he could've done better but to be fair, their goalkeeper has pulled off some unbelievable saves as well
 It's good to have him back in the team as he makes a difference but it wasn't enough tonight
"I'm disappointed for our fans tonight
 They created such a great atmosphere and we had a fine start to the game where we took it to them and created some good opportunities
 It's sad to walk away tonight having lost 4-0
 We're all very disappointed in the dressing room and we should be
 We must have a reaction
"We've made things difficult for ourselves [to quality]
 Even towards the end, there was no need to concede a fourth
 It shouldn't have been a 4-0 game
 We should've been stronger mentally towards the end to keep ourselves in it
 It made things difficult but now there's not to be a reaction
"We've just got to keep persevering
 We've been disrupted by a number of injuries and we've rebuilt the squad
 It's not surprising it'll take time, but even within that I know we can be better
 We need to use the disappointment to fuel some fire to turn things around
""They've lost convincingly
 It was a terrific start to the game, played at a really good pace and Celtic created a number of chances before they fell behind
"There's been some naivety from Celtic tonight in their defending especially, but take nothing away from Leverkusen
 Their experience has shone through, although I thought the penalty was harsh on Cameron Carter-Vickers
"Joe Hart has made a couple of saves tonight to keep the score down but having said that, Celtic are a team that makes chances, but for the Leverkusen goalkeeper
 He's made at least three terrific saves
"An entertaining game but Celtic have lost quite convincingly
"Celtic are back in Scottish Premiership action on Sunday when they visit Aberdeen, live on Sky Sports; kick-off 12pm
 They will be back in Europa League action on Tuesday 19 October when they host Ferencvaros at 3
30pm
Bayer Leverkusen face Arminia Bielefeld in the Bundesliga on Sunday, before traveling to Real Betis on Thursday 21 October in the Europa League
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