Some poor defending helped Union Berlin to victory as Celtic maintained a 100 per cent loss rate in their pre-season games ahead of their first Champions League qualifier against Cliftonville in Belfast on Wednesday night.
The main bright spot for manager Neil Lennon was that captain Scott Brown managed 60 minutes as he continues his recovery from a groin injury.
Lennon revealed before the game that his squad's injury situation was "starting to ease" but he remained without Joe Ledley, Adam Matthews, Charlie Mulgrew, Virgil van Dijk and Gary Hooper, who have all been struggling with injury.
Trialist Steven Mouyokolo was also absent meaning Celtic started with 20-year-old Australian Jackson Irvine, a late addition to the touring party, at centre-back alongside Kelvin Wilson, while Lennon had a largely inexperienced bench.
Celtic had lost 6-2 to Union's Bundesliga 2 rivals Greuther Furth on Tuesday and they found themselves behind inside 11 minutes at a packed Stadion An der Alten Forsterei, where about 20,000 fans were marking the opening of the new main stand.
Midfielder Tomas Rogic lost the ball midway inside his own half and it was helped on to Simon Terodde, who unleashed a 20-yard volley that swerved past Fraser Forster into the roof of the net.
Forster appeared to be taken by surprise by the effort but he pulled off an impressive stop moments later before Celtic improved in the latter stages of the half with Mikael Lustig heading not far wide.
But some slack defending cost the Scottish champions 10 minutes after the restart with Celtic failing to react as a half-cleared corner was sent high back into their box for the unmarked Soren Brandy to stab home.
Union had a goal disallowed for offside before Irvine and Stokes got efforts on target, but Celtic conceded another soft goal three minutes from time.
The visitors, who also lost to Ukrainian side Sevastopol and Romanians Cluj in Germany, failed to pick up at a short corner and the ball was crossed for Damir Kreilach to rise above substitute Amido Balde and head home from close range.