The 18-year-old has reportedly been told he will not be selected for either the first team or age-group sides until he signs a new contract.
Chelsea bosses are understood to have believed a new deal had been verbally agreed with Acheampong before he backed out, effectively giving an ultimatum over his future.
He did not travel with the squad, which arrived in Athens on Wednesday ahead of the team’s Conference League meeting with Panathinaikos.
“I’m embarrassed for Josh,” Maresca said.
“This kind of game could be an opportunity for him. I spoke with him personally about three weeks ago about his situation. The chat we had was very positive.
“It’s a shame because I really think Josh could potentially be a top player. He is still 18, so many things can change for him, but potentially he could be very important for Chelsea.
“He is so important to the club that our intention is not to lose him. The first thing we have to do is find a solution to his contract, his future.
“It’s a bad situation at this point for him. I don’t know if the people who are trying to solve the problem are helping him or not.”
Achiampong, who is an England Under-20 international, made his professional debut as a second-half substitute during the 2-0 win over Tottenham at Stamford Bridge last season.
His only appearance this campaign came in the 5-0 Carabao Cup win against Barrow, but with Maresca rotating their squad heavily between the league and cup competitions, the game in Athens could be a chance for him to feature.
“I really like Josh and would love to have him here with us,” Maresca said. “But it’s not possible.”
Victory over Panathinaikos would maintain Chelsea’s perfect start in Europe since beating Belgian side Gent 4-2 three weeks ago, but the game at the Olympic Stadium will be particularly poignant for Maresca.
It was at this stadium that his Sevilla team played their first game since the death of defender and team-mate Antonio Puerta, aged 22, in August 2007.
Puerta collapsed on the pitch during a match against Getafe. Although he was resuscitated and was able to walk off the field unaided, he went into cardiac arrest and died in hospital three days later.
The following week, Maresca and Sevilla played away in the Champions League against AEK Athens.
“It’s a mixed feeling,” the Italian said of his return.
“The season I spent here in Greece (with Olympiakos) was very good. At the same time I was here unfortunately after losing a friend on the pitch in Sevilla.
“The first game we played after that, one of our teammates died, Antonio Puerta. Feelings are mixed. Every time I come back here, I feel a little sad.”