City's humbling by Bayern Munich earlier this month has effectively left them in a battle with CSKA Moscow for second place in Group D.
City and CSKA both have three points - three less than reigning champions Bayern - courtesy of their victories over Czech minnows Viktoria Plzen.
With the German giants now expected to brush aside Plzen to take firm control of the group, City's back-to-back clashes against CSKA look pivotal.
Progress from the group, after failures in the previous two seasons under former boss Roberto Mancini, is the very least expected of Pellegrini.
And the Chilean is well aware of the importance of the trip to the Russian capital, feeling the game equates to the first half of a two-legged battle to go through.
"I think that 50 per cent of the chances to go into the second round of the Champions League will be playing next Wednesday in Russia," he said.
The game comes amid a testing spell of away games for City, four days after a trip to West Ham and four before they travel to title rivals Chelsea. They follow that with a journey to Newcastle in the Capital One Cup.
Changes are likely but Pellegrini has defiantly insisted he is not prioritising one game over another.
He said: "For all the managers the best thing is to keep an 11 starting when you are winning and playing well.
"But I think we must be a competitive team all year and it is very difficult to keep the same 11 starting.
"There is no priority. For me it is very important for the team to qualify for the round of 16 in the Champions League but also the Premier League is very important."
There is uncertainty over the state of the pitch City will be playing on next week.
CSKA switched their last Champions League home against Plzen to St Petersburg, around 400 miles away, after heavy rain ruined an already well-worn surface at their Arena Khimki home.
Dinamo Moscow also play their home games at that stadium while the ground has been used by Spartak Moscow and Russia Under-21s as well this season.
There were suggestions City's game could be relocated to Krasnodar, 800 miles south of Moscow, but UEFA last week told City the Khimki Arena had been declared fit for use.
Yet despite some remedial work in the past fortnight, recent reports suggest the pitch still lacks grass.
City will not need reminding of the one poor surface they have encountered this year, in Hong Kong in pre-season, when Matija Nastasic suffered an ankle injury that ruled him out for a month.
The Russian problem has arisen because the Luzhniki Stadium, Moscow's biggest arena and regular Champions League venue, closed after this summer's athletics World Championships for renovation ahead of the 2018 World Cup.