Charlesworth admitted the timing of Stajcic's departure -- first reported by ESPN on Wednesday -- blindsided him, but claimed it was not linked to doubts over the future viability of a franchise he pledged to continue backing whilst the search for a new majority shareholder continues.
Charlesworth gave the former Matildas coach due credit for transforming Central Coast from wooden spoon specialists into contenders during his two-and-half-year year tenure.
But he speculated that Stajcic might have baulked at the prospect of another season working under tight budgetary constraints in Gosford.
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"I'm not sure of the real reason, though I suspect he wanted a bigger budget to play with," Charlesworth told ESPN.
"He's had a very good season for us. I knew he was thinking of moving on but I didn't know for sure until a couple of days ago that he was going to act on it. In fact, I thought maybe he wouldn't, quite frankly.
"We haven't had time yet to analyze who we'll bring in but we'll want someone in place fairly quickly.
"It was Alen's call -- he decided to go. It's disappointing but we've overcome much bigger issues.
"Hopefully he gets a good club next and gets the right opportunity to prove himself again. It's about his CV and he has a good one.
"We've got bigger challenges than this -- but where there's an issue there's also an opportunity.
"We'll give an opportunity to a new coach now who possibly might not have an opportunity elsewhere."
Stajcic has already been linked with the vacant Western United job, with his assistant Nahuel Arrarte also quitting Gosford.
He said in a statement: "After a long period of discussions and reflections I have decided that I want a new challenge and to go in a new direction in my coaching journey.
"I came to the club in 2019 when it had just one win in 22 matches and when I was also at a crossroads in my career.
"I leave with a lifetime of bonds, friendships and memories."
Charlesworth proclaimed his commitment to another Mariners reinvention, as he looks to appoint a seventh coach in as many years.
"I don't think Alen's departure is tied to the future of our club at all -- people know I've been looking for investors for 10 years now," he added.
"I've always been committed to the club -- my No. 1 priority is to try and find a solution that creates a sustainable business on the Central Coast.
"I think most people realize that -- Alen Stajcic certainly knew that because we discussed it."
Charlesworth was reportedly in dialogue with Manchester United over partial sale earlier this year before talks stalled.
Negotiations with Gosford Council are ongoing over taking over the management rights at Central Coast Stadium, an outcome Charlesworth views as pivotal to maintaining the club's roots in the region.
"We're trying to reach an agreement that will create more stability," said Charlesworth. "Until that happens, and investor comes in, then it's business as normal.
"At this point, there are no immediate buyers around the corner."
Charlesworth hopes to keep the core of the squad together.
"Possibly one problem we had this year was that we lacked strength in depth,"' he said. "But we've given a lot of chances to young players in recent years and it's been pleasing to watch them develop."