"A win in Belfast would take us a huge step towards the World Cup in Brazil," said the former England manager, who replaced Dick Advocaat as Russia boss following the team's poor showing at Euro 2012.
After they made a flying start to their Group F qualifying campaign by winning four matches on the trot without conceding a goal, the Russians lead both second place Israel and Portugal in third by five points.
The 97th-ranked Northern Ireland are a distant fourth with three points from four games and have only an outside chance of clinching a runners-up spot having failed to beat either Luxembourg or Azerbaijan at home.
Such is their dominant position, Russia can afford to lose June's reverse fixture against Portugal, considered their main challengers for top spot, and still remain in the driving seat.
They will have home games against minnows Luxembourg and Israel in September, before facing Luxembourg again and Azerbaijan in their final two qualifiers the following month.
ARSENAL GUESTS
"A positive result against the Irish would allow us to go to Portugal without too much pressure," Capello said. "Portugal would then feel all the pressure as they need to make up ground on us. That's why I say the Belfast game is the most important."
The Russians, who also face five-times world champions Brazil in a friendly at Chelsea's Stamford Bridge stadium on Monday, have been put through their paces at Arsenal's training ground in London this week before making the short trip to Belfast.
Northern Ireland have never beaten Russia, or their predecessors the Soviet Union, in five previous attempts, although they held the Soviets to draws in qualifiers in Belfast in 1969 and 1971.
Manager Michael O'Neill is still waiting for his first win after eight games in charge, but has received support from his predecessor Nigel Worthington.
"Knowing Northern Ireland these are the type of matches where they may get a win and [O'Neill can] get the monkey off his back," Worthington said.
"I am sure he is working very hard and it is just a matter of time before he gets that first victory.
"But it is not an easy job because you have limited resources in terms of players."
Capello is expected to keep faith with his first-choice goalkeeper Igor Akinfeyev and is likely to pair Dynamo Moscow's Alexander Kokorin with misfiring Zenit St Petersburg striker Alexander Kerzhakov in attack.
Akinfeyev, 26, will be looking to keep a 10th consecutive international clean sheet and extend the all-time record for Russia and the Soviet Union.
