The Republic head for Sweden on March 22 and entertain Austria at the Aviva Stadium four days later as the three sides chasing Group C leaders Germany jockey for position in the race for second place.
Sweden are the current occupants with seven points, one more than Giovanni Trapattoni's men, while the Austrians lie in fourth on four points with all three teams having played three games.
However, the Swedes went to Germany on October 16 last year, four days after Ireland were trounced 6-1 on their own pitch by Joachim Low's side, and returned with a point after a remarkable 4-4 draw.
Asked if next month's fixtures are likely to be decisive, Whelan said: "Yes, I think so.
"We know Germany are probably the favourites - well, they are the favourites - so everyone is fighting for the second spot.
"Obviously, the results that Sweden have been getting, it's going to be tough, but we can only go out and do our best.
"If we get the right result, it gives everyone a lift and something to look forward to, so that's what we are hopefully going to do."
Trapattoni and his players will head for their showdown in Stockholm in positive mood after Wednesday night's 2-0 friendly victory over Poland in Dublin.
The Italian rang the changes, partly through necessity with Richard Dunne, Seamus Coleman, Keith Andrews and Darron Gibson injured while skipper Robbie Keane and central defender Darren O'Dea were excused duty.
Millwall's David Forde started in goal with number one Keiren Westwood's lack of club football with Sunderland a growing concern for his international manager, while Ciaran Clark, Greg Cunningham, Robbie Brady, James McCarthy, James McClean and debutant Conor Sammon started.
There were impressive cameos too from Norwich's Wes Hoolahan and Derby's Jeff Hendrick, who combined for the former to cement victory with his first senior international goal after Clark had opened his account before the break.
Trapattoni for once had admitted in advance that the result was secondary on a night when he wanted to look at his options, but in the event, he got both as Ireland ended a run of two successive home defeats in spirited fashion after having to weather an early onslaught.
Whelan said: "People were saying it was only a friendly, but if we had lost, it would have been a different story and different headlines.
"We knew it was massive. We have got a great bunch of young lads coming through and it's been a big turn-around from the last couple of years because we have had some massive players who have retired from international football.
"But the new lads have stepped up and it's massive now for next month."
Defender John O'Shea, captaining the side in Keane's absence, admitted the new players, and 30-year-old Hoolahan in particular, had given the manager something to think about ahead of two vital games.
He said: "It was a great goal from Wes. There might have been a suspicion of handball - I think the Gaelic Dublin team better watch out.
"It was a great finish from him, though. The quality of Wes, that's what he is capable of.
"He came on and he has given the manager a tough decision now for March. He came on and made a massive impact, he did what he had to do.
"We probably passed it a little bit more because obviously the size of Wes, we were looking in the second half to keep the ball a bit better and bringing Wes on helped us to do that - but we can do a lot better at that as well.
"He took his goal fantastically. The manager especially thinks of people when they are scoring goals, it definitely opens his eyes, and we will wait and see. But there are bodies to come back as well."