“Being in the position that I can afford to Kris Commons out is a huge step forward for us”
"Obviously I'd like him to be involved and if the specialist thinks he can be then I can bring him in but it looks unlikely at this time."
The news on McGregor, who sustained a groin injury against Australia, was more positive.
"Allan feels he will be fit to play," added Levein. "There is still a small element of doubt but the good thing is that we gather on Sunday so we'll have plenty of time to assess him.
"We're talking about an experienced player in his late 20s who knows his body and he's confident he will be fit."
Ross McCormack keeps his place in the squad after scoring as a substitute against the Socceroos.
He will compete for a forward berth with Jordan Rhodes, who scored his first international goal against Australia, Steven Naismith, Jamie Mackie, and Kenny Miller.
Miller, who features in the squad for the first time since moving to Vancouver Whitecaps, may captain in the side in the absence of Darren Fletcher, who continues his comeback from a prolonged illness-related absence.
The Manchester United midfielder will, however, train with the squad, as he did before the Australia match.
Wigan defender Gary Caldwell is in line to win his 50th cap and is also a candidate to wear the armband.
Interview - Scotland manager Craig Levein
Barry Bannan drops out but his Aston Villa team-mate Alan Hutton remains in the squad despite not playing for his club this season.
Southampton left-back Danny Fox keeps his place, with full-backs Phil Bardsley and Steven Whittaker absent through injury and Lee Wallace overlooked for the opening Group A matches.
Wallace's Rangers team-mate Ian Black, who was booed by some fans when he made his Scotland debut as a substitute against Australia, is also omitted.
Commons, who has scored four goals in six games this season, has not featured for Scotland since withdrawing from the squad ahead of the May 2011 matches against Wales and Republic of Ireland.
"We are so strong in the forward and wide areas that I didn't feel that any of the guys who have been in the squad recently deserved to be left out," said Levein.
Brown and Commons have been omitted from the Scotland squad
"We're very fortunate to have James Forrest, Jamie Mackie, Ross McCormack, Shaun Maloney, Kenny Miller, James Morrison, Steven Naismith, Matt Philips, Jordan Rhodes, Robert Snodgrass.
"All of those guys can play in the same positions as Kris Commons. I feel I've picked the best squad available.
"It's good to see Kris has started the season particularly well but these guys have done exceptionally well for me. Being in the position that I can afford to Kris out is a huge step forward for us.
"The one I feel most sorry for is Craig Mackail-Smith. He doesn't deserve to be left out but the fact is competition is really strong just now."
Matt Gilks (Blackpool), Allan McGregor (Besiktas), David Marshall (Cardiff City)
Christophe Berra (Wolverhampton Wanderers), Gary Caldwell (Wigan Athletic), Daniel Fox (Southampton), Grant Hanley (Blackburn Rovers), Alan Hutton (Aston Villa), Russell Martin (Norwich City), Charlie Mulgrew (Celtic), Andy Webster (Heart of Midlothian)
Charlie Adam (Liverpool), Don Cowie (Cardiff City), Graham Dorrans (West Bromwich Albion), James Forrest (Celtic), Shaun Maloney (Wigan Athletic), James Morrison (West Bromwich Albion), Matt Phillips (Blackpool), Robert Snodgrass (Norwich City)
Ross McCormack (Leeds United), Jamie Mackie (Queens Park Rangers), Kenny Miller (Vancouver Whitecaps), Steven Naismith (Everton), Jordan Rhodes (Huddersfield Town)
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