The Ibrox club defeated the likes of Lyon, Borussia Dortmund and RB Leipzig on route to the final and Gers legend Butcher believes the squad took the surprise Belgian outfit too lightly.
He is adamant the result will serve as a real wake-up call to the current squad, but remains upbeat about their chances of progressing to the play-off round against either PSV Eindhoven or Monaco later this month.
Butcher said: “In football, you don’t get the time to sit back, admire the view and think, ‘We’re a bloody good side’, as there’s always someone there waiting to knock you down.
“Rangers did really well to reach the final last season, but I know from personal experience how that can affect your mentality as a player.
“In my first season at Ibrox we won the championship and the League Cup, and I was expecting that to be the case every year, but we had a rude awakening when we ended up losing the league the next term, although me breaking my leg didn’t really help matters!
“Sometimes players need to look in the mirror and give themselves a slap in the face. That result on Tuesday was a slap all right.
“Those lads will be gutted, but they are lucky they have the chance to put things right next week. Two years ago, during lockdown, these qualifiers were one-off games and Rangers would be out.
“Rangers had a lot of heroes last season, they will need a few more in the return leg.”
Van Bronckhorst will hope to call on key duo Alfredo Morelos and Ryan Kent both available for the second leg at Ibrox next Tuesday after several new signings struggled to grasp their opportunity in Leuven.
Talisman Morelos is expected to win his fitness battle after a five-month absence due to a thigh injury, while winger Kent picked up an ankle problem against Livingston last weekend but the injury is not thought to be serious.
Butcher is relieved the tie remains alive but acknowledges Rangers must up their game significantly in front of their home crowd, insisting the first goal will be vital.
He added: “The game was the main concern and it was a short, sharp reminder of the tough challenges they face.
“But the Belgian side won’t have experienced anything like the emotion, noise and passion generated at Ibrox on these big European night.
“We shouldn’t forget that the situation could be a lot worse. After that display in Belgium, they probably should be out of the competition already, but 2-0 can be a dangerous lead during a game, never mind after a first leg.
“The first goal next week will be crucial, although Rangers are helped by the fact that away goals no longer count double.
“However, if they can score early doors then that could be a real game-changer. It would lift the fans to another level and it could turn out to be a very memorable night.
“Personally, I’m not too worries by the scoreline. Rangers going in front would put the frighteners on Union Saint-Gilloise, and they have the firepower to go on and get the result they need to really kick-start their season.
“Gio van Bronckhorst had a lot of new signings on show and they are nowhere near hitting their stride yet, guys like Tom Lawrence, who was fantastic at Ipswich.
“It’s early days for this team, but the crowd can play a huge part in carrying them over the line next week.”
With city rivals Celtic guaranteed to earn a lucrative £40million for securing a group stage spot, Butcher knows Rangers must join them as he outlined the prestige of the compettion as another added motivation.
He stated: “That’s a huge incentive for everyone involved. Players don’t think about earning money for their clubs, it’s about pride for them.
“Rangers have done well in the Europa League since Steven Gerrard came to the club, but they need to be at the top table now. They don’t want to suffer the ignominy of dropping down to the lesser competition.”