Owing to injury and match fitness concerns, the 21-year-old is having to wait longer for his first appearance for the Saints, five months after his acquisition.
He joined the Premier League side from La Liga outfit Real Valladolid.
"He [Salisu] came to us with a medical issue and we knew he probably won't play until October," Semmens told the BBC.
"From our perspective, we made that decision [to still go ahead with the transfer]. That decision is on me, it's not on him or [Southampton manager] Ralph [Hasenhuttl]. We made that decision because we believe he's going to be a superstar in the future.
"We expected him to be available from the middle of October and in November he was getting on track, then he had a setback that probably pushed him into December.
"Just when he was ready, just when he was training every day and doing really well he picked up a muscle injury, I think after the Fulham game. So he was ready to play and if fit, Ralph would have selected him but he picked up a muscle injury. He's not too far away.
"I think we need to be really clear here: we are exceptionally happy with him as a person and a character. He's physically great and fits our system and Ralph loves working with him and he'll be a great player in our future."
Despite signing his Southampton deal in August, Salisu joined the club's pre-season training a month later, in the week of the commencement of the Premier League.
He made the matchday squad for the first time in December, going on to make the list five more times before going missing again due to his latest injury.
"Again as you referenced he's 21, he had a difficult injury problem, he's from a foreign country he's new in this country in a lockdown environment I think we have to give him a shot," the CEO explained.
"And again if you listen to Ralph's press every week as I do, he's said the same thing: we probably won't buy players in the future that will be at the level to help us from day one.
"They will need a few months possibly in the season to get to the level where they're a top-level Premier League player.
week in the Premier League it's going to cost you a lot of money.
"So our model is to take the bright young things from around Europe and improve them and I promise you with no doubt that he [Salisu] will get to that level."
Salisu's injury predicament has also prolonged his anticipated Ghana debut, having turned down his first invitation for a double-header against South Africa and Sao Tome and Principe in 2019. Read Full Story