Following their opening defeat in Milan, the Scottish champions were beaten 1-0 by Barcelona on Tuesday, with captain Scott Brown sent off for a petulant kick at Neymar.
They now take on Ajax - who are one point better off in Group H - over two games on 22 October and 6 November and Commons accepts it is do or die for Celtic.
He said: "I think we need to win our two home games and then we need to go on the road and try to pick up a win and that's going to be a mammoth task looking at the games we have left.
"It's a similar sort of feeling as after the Milan game.
"We feel as if we've put in a lot of hard work, but ended up getting an unlucky decision from the referee and your back is against the wall from then and you've got 20 minutes to try to stop the best team in the world from scoring.
"You're practically up against the impossible when you have 11 men, not only
to get the ball but to create chances.
"For 60 or so minutes we defended magnificently and kept them at bay. I can't think of Fraser Forster having to make a save in the first hour.
"But then Scott gets sent off and obviously our game plan has to change - a little bit of lady luck would have been nice."