Palmer sustained the injury in August, but returned to action with back-to-back goals against Brentford and Bayern Munich.
The 23-year-old England international, however, aggravated the injury during Chelsea's 2-1 loss at Manchester United on September 20.
"I was wrong (about Palmer). Unfortunately he needs to be out for six more weeks," Maresca told reporters on Friday.
"We try to protect Cole as much as we can. The most important thing is when he comes back, he is fully fit.
"The medical staff are not magicians. We hope six weeks are enough. We need to see it step-by-step. For sure, he's going to be okay.
"He looks relaxed. He has tried to recover from the injury and is doing all the therapy."
Palmer, who has scored two goals in four matches across all competitions this season, is set to miss the Forest fixture, a Champions League tie against Ajax and another Premier League match versus Sunderland.
Chelsea sit seventh with 11 points from seven matches, trailing leaders Arsenal by five points.
Replacing Palmer is difficult
"First of all, to replace Cole, it's difficult. Cole is a very important player for us, one of the best in the league ..." Maresca said.
"It depends on the game plan we have. We don't have another player like Cole. Cole is unique."
Chelsea face additional challenges in defence, with French centre back Benoit Badiashile ruled out until December due to a muscle problem.
Centre back Levi Colwill is already sidelined with an anterior cruciate ligament (ACL) injury.
"We are going to wait for him (Badiashile)," Maresca added.
"Moi (Moises Caicedo), Enzo (Fernandez) and Pedro (Neto) did not take part in the session yesterday. We will see if they can train today. Reece (James) is okay, he is fully fit."
Maresca will watch his side take on Forest from the stands following his one-match touchline ban and fine of 8,000 pounds ($10,744.00) after he admitted a charge of misconduct during the 2-1 win over champions Liverpool.
Chelsea's winner in the fifth minute of stoppage time sent the Italian into a frenzy and he raced along the touchline at Stamford Bridge to celebrate wildly with his players, earning a second yellow card and dismissal from referee Anthony Taylor.
"It was a big moment for many reasons," Maresca said. "Since I joined the club, we never won a game at home in the last minute. It was against the champions of England.
"It was an important moment, for sure. For me, personally, it was a normal reaction in the moment."