Celtic got a taste of their own medicine in Germany as they came crashing back down to earth with a thumping Champions League defeat against Borussia Dortmund.Brendan Rodgers' side had won their previous 17 matches and scored 33 goals this season before arriving at Signal Iduna Park in confident mood, fresh from a 6-0 win over St Johnstone on Saturday and a 5-1 victory over Slovan Bratislava in their Champions League opener.But they found the step up in class too much to handle as they were overrun by last year's runners-up, not helped by the absence of centre-back Cameron Carter-Vickers and some slack passing throughout their team.Celtic were briefly level when Daizen Maeda cancelled out Emre Can's early penalty but Karim Adeyemi bagged a first-half hat-trick and Serhou Guirassy also scored from the spot before the interval.Guirassy and Felix Nmecha piled on the misery in the second half as Rodgers found himself presiding over another Champions League thrashing.Celtic's heaviest European defeat remains a 7-0 loss in Barcelona during Rodgers' first spell in charge and he also suffered a 7-1 defeat against Paris St Germain and a 6-0 loss to Atletico Madrid.Liam Scales started in the heart of the Bhoys defence, with fellow Irish international Adam Idah coming on as a second-half substitute.The warning signs were evident from early on when Adeyemi had numerous options but could not find a pass or shot, and Can provided the opener in the seventh minute after Kasper Schmeichel brought down Jamie Gittens after the England Under-21 winger had burst beyond Celtic's back four.Celtic were level inside two minutes when Maeda bundled home Arne Engels' cross but Dortmund regained the lead in the same timescale.