Huddlestone's close-range howler was "easier to score" according to Tigers boss Bruce, who had to settle for a 0-0 draw on their European bow in Slovakia on Thursday.
Last year's FA Cup runners-up were making their first competitive continental appearance in the opening leg of their third-round tie in Zilina, and would have won if Huddlestone had his shooting boots on.
The midfielder saw his 61st-minute penalty saved before contriving to hit a point-blank follow-up against the crossbar and into the stands.
Huddlestone has missed from the spot before, including in a failed Europa League shootout for Tottenham against Basel in 2013, but Bruce was sure he had the right man.
"You'd have put the house on that Tom would score," he said.
"It was easier to score. We've all missed penalties, we've all missed big chances in front of goal, you don't intend to do it.
"But we've missed our last three or four I think so we better start practising because we're not very good at them."
Huddlestone himself took to Twitter to say: "Can only apologise to every (sic) for a lapse in concentration and thinking I had scored the rebound before I had. Ridiculous on my behalf.
"On a positive note, the lads worked hard and defended well throughout deserved our clean sheet. All to play for next week."
Although the Tigers were favourites to win the away leg, they still have next Thursday's game at the KC Stadium to secure a place in the play-offs.
And that is enough for Bruce, who was concerned his side might not yet be match sharp so soon after their summer break.
"We needed a game like this," he added. "I was always a bit worried we'd be undercooked a bit, it was always a big ask.
"We've only been back at work two-and-a-half weeks.
"We knew it was going to be difficult whatever team you play, especially in the middle of pre-season. But overall we'll be better for this game.
"The one thing you've got to do is take your chances when they come your way. Unfortunately we haven't taken them and we had one or two really great opportunites that would have made the second leg more comfortable."
Bruce also explained his decision to start with summer signings Robert Snodgrass and Tom Ince on the bench.
A familiar starting XI was given first chance at Trencin before the new arrivals entered the fray with 25 minutes remaining.
Bruce said: "I decided the boys who got us here deserved it.
"We'll have a bit of competition for places because they (Snodgrass and Ince) did well enough to deserve a start when they get going but a week is a long time in football, so we'll see.