Allen will miss the crunch clash in Azerbaijan on November 16 after being booked in the 1-1 home draw against Croatia on Sunday.
The influential Stoke midfielder was cautioned four minutes from time, and his third yellow card of the competition means he must sit out the penultimate qualifier in Baku.
"In hindsight Ben (Davies) was around on my cover and it was a tackle I probably didn't need to make," Allen said after Gareth Bale's 33rd Wales goal had earned Wales a share of the spoils against the World Cup finalists.
"It was an instinctive thing and unfortunately when you're on a tightrope of two yellows there's always that risk.
"Obviously I can't play a part in Azerbaijan, which is hugely disappointing.
"But I'm confident that the lads can take care of business, and hopefully I will be back in the fold for the last one with everything to play for."
Wales' final group game is at home to Hungary three days after their 6,000-mile round trip to Baku.
Ryan Giggs' side would definitely leapfrog Hungary, whose victory over Azerbaijan on Sunday moved them into second place, by winning their last two games.
If they were to do that, Slovakia would have to win their final two games – away in Croatia and at home to Azerbaijan – to deny Wales a place at next summer's European Championship.
Wales' cause is helped by the fact that leaders Croatia need a point to secure their qualification when Slovakia visit Rijeka on November 16.
"We're still well in this race," Allen said.
"We'd have loved to have come away from this camp with the two wins, but away to Slovakia and home to Croatia are not easy games.
"To come away from them unbeaten we've given ourselves a chance by winning the last two games.
"It's no easy feat but we've got the confidence and belief to do it. We've got every chance now."
Wales have been playing catch-up in the group since summer defeats in Croatia and Hungary.
But Allen feels progress has been made in holding Slovakia and Croatia to 1-1 draws during the last week.
"I think the two performances have been a big improvement," he said.
"You go back to the summer, two difficult away games but we were disappointed to come away with nothing.
"We gave ourselves a bit of a mountain to climb and we were in a difficult situation.
"But we've turned it around a little bit. There's plenty of work to be done of course, but we wanted to make sure that we're going into next month with every chance of qualifying.
"To come from a goal down against a top team like Croatia is a good result – and we're hoping it's going to play a big part in us qualifying."