Marley Watkins scored the only goal of the game as Kilmarnock sensationally sent holders Celtic spinning out of the Viaplay Cup at Rugby Park.The big striker converted a Danny Armstrong pass in the 59th minute of a pulsating last-16 tie in which the home side were well-organised and disciplined.Killie withstood Celtic's relentless pressure thereafter to complete a remarkable early-season Old Firm double, having beaten Rangers 1-0 on the opening day of the cinch Premiership.The 1-0 defeat was a bitter and unexpected blow to the treble winners who have won the trophy six times in the previous seven seasons.Kilmarnock boss Derek McInnes has certainly got his side off to a flying start this season.After taking four points from six in their opening two Premiership fixtures, Killie were unchanged and confident.With Cameron Carter-Vickers and Stephen Welsh injured, Brendan Rodgers gave a debut to 23-year-old Sweden defender Gustaf Lagerbielke while there was also a first start for summer signing Odin Thiago Holm.Lagerbielke conceded an early corner when goalkeeper Joe Hart failed to collect his pass back and Killie defender Corrie Ndaba flicked a Danny Armstrong delivery just over the bar.Second later at the other end, Hoops striker Kyogo Furuhashi stretched to knock a Holm cross past the post before midfielder Matt O’Riley’s tame shot from inside the box, after a terrific set-up by Furuhashi, was gathered by Kilmarnock keeper Will Dennis.The ball continued to speed from end to end.Celtic missed a chance in the 29th minute when Armstrong was robbed by Furuhashi near the touchline and the Japan international raced clear only to see his low drive blocked by Dennis.Killie had an opportunity in the one minute of added time but