The Cowboys half returned from a 10-week absence with an ankle problem in last week's 36-16 loss to the Dolphins only to injure his other leg.
In a statement released on Friday the club said the 25-year-old had "suffered a high-grade peroneal muscle avulsion tear" and was eyeing a late season return.
"Tommy has been ruled out for the next three weeks but he'll be back after the bye," coach Todd Payten said.
"He's torn a muscle in his shin on his good leg. He did it just after halftime (against the Dolphins) and managed to finish the game, which again highlights his toughness."
Dearden is set to miss Saturday night's home clash with the Broncos and games against Sydney Roosters (home), Gold Coast (away) and Parramatta (away). The eighth-placed Cowboys then have the bye and two home games against Wests Tigers and Canberra to finish the season.
The Cowboys have recorded 10 wins for the season and will need to win three of their remaining six matches to play finals.
"(Dearden) is disappointed but he's also pragmatic about what needs to happen when he's going after his rehab," Payten said.
"We've had plenty of practice without him. We've had seven games, we won four of those. Our backline is going to line up the same as (against) Penrith and Manly."
The Cowboys won both of those games with Jaxon Purdue and Jake Clifford in the halves.
Clifford was demoted to the bench when Dearden returned last week against the Dolphins but had been recalled for the Broncos match anyway. Purdue, who can play anywhere in the backline, will shift from centre back to five-eighth.
The Broncos must win all seven of their remaining games to play finals and will be looking to repeat their form from the 14-12 away win over the Panthers last week when fullback Reece Walsh took control in a man-of-the-match display.
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