The season begins in Bahrain next week and Hamilton, hoping to win back the title from Red Bull's Max Verstappen after a controversial end to the 2021 season, took the underdog position.
"At the moment, I'm sure everyone can figure that out, we are not the quickest," the 37-year-old Briton told reporters on the final day of pre-season testing at the Sakhir circuit.
"I think Ferrari look to be the quickest and perhaps Red Bull and then maybe us or McLaren. We're currently not at the top."
Asked whether he had any concerns about his ability to fight for the championship, Hamilton said it was too early and expressed confidence in his team's proven ability to work through any problems.
Mercedes have won the last eight constructors' championships.
Hamilton said the new car had shown decent reliability and the team had got through their test programme but it was clear there remained much to do.
"At the moment I don't think we'll be competing for wins," he said.
"There is potential within our car to get us there, we've just got to learn to be able to extract it... we have some hurdles to overcome.
"Next week we'll get a much better showing of our pace but I think people will be surprised, maybe. People keep talking about (whether) we're talking ourselves down or not but it's a bit different this year."
Mercedes's rivals have taken such declarations with some scepticism in the past.
Last year Mercedes had a troubled pre-season, completing the fewest laps of any team, but Hamilton still won the opening race in Bahrain.
In 2019, Mercedes had looked up to half a second off Ferrari's pace in testing and then finished one-two in Melbourne.
This season has radically revised rules and Hamilton, the sport's most successful driver of all time with a record 103 wins, said it felt different.
"I think we have far bigger challenges this time," he said. "It will take a little bit longer. From what I'm told, we have a considerable amount of pace to find."
AlphaTauri's Pierre Gasly, who followed the Mercedes closely on Saturday and observed Hamilton's struggles with the car, was prepared to agree.
"I think they do have some work to be fighting right at the top, but it's only testing," said the Frenchman.