The ramifications of the Hoops' 2-1 aggregate play-off defeat to Maribor - sealed by Wednesday's 1-0 second-leg reverse at Parkhead - are still being felt as it meant not only the loss of around £15million in potential revenue but also loss of face, given that the Scottish champions had two chances to reach the lucrative group stages.
Legia Warsaw beat Ronny Deila's side 6-1 on aggregate in the third qualifier, but Celtic were reinstated when the Polish club were found to have fielded a player who should have been suspended in the second leg at Murrayfield.
Chief executive Peter Lawwell explained the club's financial situation and reiterated transfer policy to disgruntled fans in a series of interviews on Friday in which Deila was given a vote of confidence.
The Norwegian manager will have to try to recover the club's reputation in the Europa League, where they were drawn against Austrian side Salzburg, Dinamo Zagreb of Croatia and Romanian outfit FC Astra.
However, McGregor, who signed a new five-year deal on Friday, maintains the immediate priority is the Premiership.
"It is back to league business and we have got to take care of that," the 21-year-old said.
"I don't think it will be difficult.
"Obviously it was a hard knock we took during the week but we have dusted ourselves down and it is back to the league.
"All the boys are focused and ready to go."
McGregor, whose displays this term have seen him called up to Gordon Strachan's Scotland squad for the Euro 2016 qualifier against world champions Germany in Dortmund on Sunday week, is aware that newly promoted Dundee are showing good early-season form.
Paul Hartley's side are unbeaten in four games, with three draws and a win taking them into sixth place..
"Dundee have started well," he said.
"Obviously they have just been promoted and they have a new squad so they will be flying and confidence will be high.
"It is going to be a tough place to go and try to get a result so we need to be focused."