That would be a month later than the low-cost carrier currently expects.
Boeing is scrambling to finish software changes and put them in front of government regulators for review to get the manufacturer's bestselling plane flying again. Two fatal crashes within five months of one another killed 346 people and prompted a worldwide grounding, now in its eighth month.
Boeing has estimated it would receive this approval in the fourth quarter, but regulators say they don't have a firm timeline.
The approval process has been delayed several times as Boeing seeks to address additional concerns of regulators. "We still anticipate submitting that certification package to the FAA in the September time frame," Boeing's CEO Dennis Muilenburg said in August.
Boeing's board last week replaced Muilenburg as chairman so the CEO could focus on the 737 Max grounding, which has dragged on longer than many airlines anticipated.
"The FAA is following a thorough process, not a prescribed timeline, for returning the Boeing 737 Max to passenger service," the FAA said in a statement. "The FAA will lift the aircraft's prohibition order when it is deemed safe to do so. The FAA is continuing to evaluate Boeing's software modification and is still developing necessary training requirements."
A number of steps, including simulator testing and a certification flight and training, updates to training and operating standards, as well as training for the pilots, which could take 30 days, are yet to be completed, Southwest Airlines Pilots Association President Jon Weaks said in a letter to members.
Airlines say they will need at least a month before the jets can reenter their fleets once they have regulators' approval.
A Southwest spokeswoman said the company expects it to take around 120 hours to remove each plane from storage and that it plans to fly them hundreds of hours before the carrier resumes service 737 Max planes resume service.
"We continue to assess return to service timing based on information from Boeing and FAA but do not currently have a target date that we are working toward," said Southwest spokeswoman Brandy King said in a statement, adding that based on the information the airline has now, the planes are out of Southwest's schedule until Jan. 5.
Southwest had 34 of the 737 Max planes in storage at the time of the grounding.
The letter from Swapa, as the union is known, said some steps in the process that Boeing, airlines and regulators have to follow may occur concurrently and save time but the union "feels that these timelines are the best-case estimates and remain fluid."
"As of today, we are looking at probably a February timeframe to say the least," said the union's letter.
Swapa earlier this month sued Boeing in a state court in Dallas, saying the manufacturer rushed the plane to market and that its member pilots have lost out on about $100 million in income. Boeing said the lawsuit was "meritless and will vigorously defend against it."