The Hatters had forked out a club record fee of £1.3m to HNK Rijeka for the Croatian international back in August 2019, former manager Graeme Jones opening the purse strings on that occasion.
However, with the 30-year-old's contract running out in the summer, Jones opted to sell him to Bulgarian side Ludogorets before the window closed, rather than risk losing him for nothing, bringing in Aston Villa stopper Jed Steer on loan until the end of the season as a replacement to battle it out for James Shea for the number one jersey.
Speaking about the decision to let Sluga go and his hunt for a new keeper once the season has ended, Jones said: “We know with Sheasy that we have a real safe pair of hands.
“The decision to let Simon go was different.
“We’ve had a decision to make where we either renewed his contract or we had to sell him in this window, because the club paid money for him.
“We had to protect the asset, so we either offer him a new contract or sell him.
"We chose to open up the market to see if we could sell him and that’s what we did in the end.
“We love Simon, he was brilliant for us, he was brilliant for me.
"He had a tough integration but, after that he responded fantastically well.
"He was a really popular member (of the squad), but we felt we wanted to go in a different direction.
“It was a club decision, what we had to do was protect the asset, because if we let it go to the summer he walked for nothing.
“So we had to protect the club, the value and the outlay and the investment and, now, bringing in Jed has given us a good six or seven months before we have to make a decision.
“We’d like to bring in a permanent keeper, one who’s ours, that we can develop.
“Simon’s 30, which is not old for a goalkeeper, but we would like, in a perfect world, to bring in a younger one that might be with us for six, seven or eight years, if we don’t sell him.”
Speaking to the club website on Tuesday, Jones also was keen to pay tribute to the efforts of Sluga, who made 94 appearances for the Hatters, and was named Player of the Season, plus Bobbers Travel Away Player of the Season last term.
He added: "We’d like to thank Simon because I know he had a difficult start, as any player from another country would, adjusting to a life, different culture and style of football.
“Since I’ve come in he’s been excellent, his attitude, his demeanour, he has been part of a real progressive team and we thank him.
"At times if he hasn’t had the best of games, he responded well and has been a big part of the goalkeeping department, so we would like to wish him well.”