Wales take on the Republic of Ireland on Friday four points adrift of the Group D leaders.
But Rush feels the presence of Real Madrid striker Bale after ankle surgery will prove the difference at the Aviva Stadium.
"It's massive for Wales that Bale is there after his injury problems," said Rush.
"He's been the most improved player in the world over the last couple of years and, as we've seen many times before, he is capable of winning a game with a moment of magic.
"Martin O'Neill is a very cautious manager and will do everything he can to stop Wales counter-attacking the Republic.
"But it is almost impossible to keep someone of Gareth's ability quiet for 90 minutes."
Third-placed Wales are still unbeaten in Group D, though their World Cup ambitions hang in the balance following draws with Austria, Georgia and Serbia.
But Bale has already scored four times in the group to move within two of equalling Rush's 28-goal record for Wales.
"Records are there to be broken, I've never worried about them," said Rush, who scored the winner when Wales won a Dublin friendly in 1986 – Jack Charlton's first game as the Republic manager.
"I've always thought someone would break my record one day, and if you're going to lose it then it may as well be to one of the best players in the world.
"In fact, I'd like to see him break the record in Dublin because it would mean he's got a hat-trick!
"I never qualified for a World Cup, but I'm a fan now and I want to see Wales play at one in Russia.
"If Gareth is scoring goals then Wales are probably winning and have the best chance of being there.
"You saw how much it meant to the country last summer when Wales reached the semi-finals of the Euros."
The Republic have put themselves in a strong position to reach Russia as four of their final six games are at home.
Wales, meanwhile, follow their trip to Dublin with another tough away assignment against second-placed Serbia in June.
"There's a lot riding on this game, more for Wales than Ireland," Rush said as he turned out in a five-a-side match to promote the chance to win £3million for just £1 on The Football Pools.
"We did more than enough to beat Serbia in our last game but could not see it out.
"But there's still a long way to go in this group and teams will continue to take points off each other.
"Georgia have done that by drawing in Wales and Ireland were lucky to beat them in Dublin.
"But I think Wales need four points out of the next six to stay in with a chance of finishing top of the group."
:: Britain's first £3million footballer Ian Rush is supporting the chance to win £3m for £1 on The Football Pools. Visit: www.footballpools.com
