Manager Gordon Strachan will announce his group for the game in Dortmund at a media conference this morning and McGregor is expected to be included. The 21-year-old Celtic midfielder was due to be named in the under-21 squad for the matches against Slovakia and Luxembourg but Strachan has decided he has done enough to merit an elevation to the senior squad for the game against the World Cup winners on September 7.
McGregor made his debut for Celtic only last month but he has already scored four goals in eight appearances, three of them in consecutive Champions League away games against KR Reykjavik, Legia Warsaw and Maribor. He has been with Celtic since he was nine years old but, after being loaned to Notts County last season, he has finally been given first-team exposure by new manager Ronny Deila.
Danish media, meanwhile, have reported that Celtic will secure the services of Manchester United's No.2 goalkeeper Anders Lindegaard if they qualify for the Champions League group stages.
Lindegaard has, according to his agent, grown frustrated at playing understudy to David de Gea: the player failed to gain a place on the Manchester United bench for yesterday's trip to Sunderland.
Should Celtic overcome Maribor tomorrow night, they will launch a bid to sign Lindegaard on a season-long loan, according to Danish newspaper BT.
As tomorrow night's Champions League decider approaches, Deila confirmed the club are also trying to finalise details to bring Serbian striker Stefan Scepovic to Parkhead while work permit problems are delaying Mubarak Wakaso's move to the Scottish champions.
"There is still a lot of paperwork to do with Mubarak," said Deila. "But it is very early to talk about that [other transfers] and we have to assess all the team and see what we need. There will be no players coming in that can affect the game on Tuesday."
Deila chose to switch 10 players for Saturday's match in Inverness in a bid to keep players fresh for the match against Maribor. The Celtic frontline drew a blank against a stubborn Inverness Caledonian Thistle defence, with Deila replacing Teemu Pukki at half-time and substituting Leigh Griffiths with 20 minutes to go.
The manager believes a lack of game time may have affected their sharpness in front of goal. "You have to think about these things and if you don't play then you are going to lose your fitness. It's important to see where people stand," he added.