"I believe it will be the most important match for us," the Italian told the Rossiiskaya Gazeta daily.
"Northern Ireland have held Portugal to a 1-1 draw last year. For our team the away clash at Belfast will without doubts be the most serious test.Northern Ireland are very dangerous on the counter-attack and in dead ball situations. I expect a very tough clash on Friday."
Russia currently top Group F with a perfect record of 12 points from four matches, five ahead of Israel and Portugal, who meet in Ramat Gan, Israel, also on Friday. Capello, however, said that Russia still have plenty of work to do to book their ticket for Brazil.
"The five-point lead doesn't mean anything," he said. "We have played just four matches and the most serious clashes are still ahead. I expect tough opposition in every one of the remaining matches.
"We started well and left our rivals behind. But the qualifying tournament is a long way to go and we have no right to relax until we win our place in the World Cup finals."
Capello added that Russia would probably play their home qualifiers with Luxembourg and Israel in September in Saint Petersburg's Petrovsky stadium, the home of reigning Russian champions Zenit.
"The Luzhniki stadium will be closed for renovation, while the pitch at Moscow's Lokomotiv arena was in poor condition when we played our last qualifier there," Capello said.
"I think it's a good idea to play at Saint Petersburg in September. The arena there is very comfortable with an excellent pitch. It seems to be a good choice for us."