The midfielder signed on a season-long loan from Juventus on deadline day with an option to buy included at £26million.
However, despite making the bench numerous times, the Switzerland international hasn't made a single appearance so far but that could change on Wednesday evening.
"He has been really good,” Potter told reporters on Tuesday. “He's trained well and has been supporting his team-mates. He's disappointed not to play like anybody is but up until the weekend the results have been positive and with the performances.
"We're trying to take steps. He's in the frame for tomorrow. He is in the squad. He is available for selection so we will wait and see."
The situation is further complicated by FIFA rules because Zakaria is only allowed to play official matches for two teams in a season.
If he lines up for Chelsea tomorrow, he must complete the season with either the Blues or his parent club and will be unable to find another loan option in January.
Meanwhile, Chelsea had a partially open training session ahead of the meeting with Zagreb which showed six potential absentees for the upcoming match against the Croatian champions.
Potter explained the situations surrounding both Kepa Arrizabalaga and Mateo Kovacic and will also welcome back Kalidou Koulibaly.
"With Kepa, we're looking at probably days [out injured] and he is certainly not ready for the game tomorrow," he added. "Kovacic is precautionary with his calf and has a chance for the game tomorrow. We will make a decision later on.
"Koulibaly trained today so he was fine and he is available and in the squad tomorrow."
It remains unclear whether Carney Chukwuemeka will be in line for his first start after training alone ahead of the Zagreb match.
Reece James (knee), N'Golo Kante (hamstring) and Wesley Fofana (knee) also remain long-term absentees, with Edouard Mendy now set to make his first start under the new manager.
Potter called up Lewis Hall, Eddie Beach, Zak Sturge and Dion Rankine for his training session on Tuesday but played down a possible start for any of the youngsters.
"It isn't time to just be using young players for the sake of it," he added. "I think we have to be competitive and we have to be respectful of the competition, there are still teams looking for points for their own qualification.
"From my perspective, we still have to pick a team that can win the game. We have used a lot of young players and a lot of them have been impressive and I like the character and personality. It has been important in our start at Chelsea to get to know the academy guys."