The right-back left the Bhoys for Oakwell on loan last month.
The Swiss talent scored on his Barnsley debut this week.
The 20-year-old scored one and set up another for the Championship side.
Janko left Celtic for Barnsley at the end of the transfer window.
The right-back left the Scottish Premiership side and joined Championship outfit Barnsley.
The Scottish Premiership side made it through to the group-stages of the Champions League, but the defender's performance almost cost them dearly.
Celtic comprehensively outplayed Motherwell 5-0 at Celtic Park tonight in the Last 16 of the Betfred Cup.
A select round-up of the latest news involving Mark Warburton's Glasgow Rangers and Brendan Rodgers' Hoops.
The Scottish Premiership side are just one round away from reaching the group-stages.
The Celtic boss likened the 20-year-old to a "train".