An inexperienced Manchester United side strolled to victory at Parkhead in Celtic's final pre-season fixture.
Defender Jonny Evans struck from close range in the eighth minute as the home side started poorly and United were totally dominant as Paul Scholes struck twice after the interval.
Hoops boss Gordon Strachan could not fail to be alarmed by the performance three days before his side begin their Bank of Scotland Premier League title defence against Kilmarnock.
Strachan left Stilian Petrov out of the squad after a fresh Portsmouth bid for the wantaway midfielder was rejected, and started a number of fringe players including Mo Camara, Alan Thompson and Paul Lawson.
Scholes and Darren Fletcher were among a handful of recognised United first-team players and the pair controlled possession in central midfield from the outset.
Scholes saw a 25-yard strike tipped over by Artur Boruc and Evans volleyed United ahead from the resultant corner after Celtic failed to clear Kieran Richardson's deep centre.
Richardson then sent Lee Martin clear down the right channel but the youngster dragged his shot wide.
Celtic were toiling, but they almost levelled in the 25th minute when Thompson's low strike was blocked by United keeper Ben Foster.
Giuseppe Rossi should have doubled the lead moments later when he headed over from eight yards after a brilliant left-wing cross from Martin.
As Celtic's midfield finally began to push United back, Maciej Zurawski set up Aiden McGeady but the Bhoys' brightest spark of the first half rolled the ball wide from 12 yards.
Rossi then turned Gary Caldwell with ease on the edge of Celtic's box but Boruc rushed to block just before the interval.
Celtic had started with first-choice strikers Zurawski and Kenny Miller but neither threatened Foster's goal and both made way at half-time.
A relatively docile home support sparked into life to welcome former Hoops midfielder Liam Miller with a chorus of boos as Scholes was pushed forward.
And the former England star made it 2-0 on the hour mark when he stabbed home from two yards after Richardson drove a free-kick from the right edge of the box towards Boruc's near post.
At the other end, Derek Riordan curled in a dangerous free-kick from 40 yards out on the left which fellow sub Adam Virgo headed on but Foster dived to save.
Richardson came close with a 25-yard free-kick and another whipped corner from the wide man found the head of Paul McShane, whose header seconds after coming off the bench was beaten away by Boruc. Richardson immediately sent in another cross which Scholes headed over from 10 yards.
Boruc made a great double save after Rossi's 30-yard free-kick but he failed to hold a weak Scholes header in the 84th minute and the prolific midfielder slid home the rebound.
It was all too much for one Celtic fan who invaded the park to register his dismay.
Miller almost rubbed salt in the wounds when he burst through but shot wide as Celtic ended a dismal pre-season campaign which saw them score just twice in seven games.