Timmons went AWOL from the team unexpectedly this past weekend and did not play in Sunday's 19-17 road victory over the Los Angeles Chargers.
Timmons signed a two-year, $10 million contract in March, but he has not played a regular-season game.
ESPN's Jeff Darlington reported that Dolphins coach Adam Gase is still upset with Timmons for abandoning the team without notice Saturday night, and he is not yet satisfied with any of the explanations he has received.
Gase still wouldn't discuss details of Timmons' disappearance or his punishment on Wednesday, instead telling reporters that information will stay "in house."
So while the Dolphins have decided to suspend Timmons indefinitely, sources told Darlington that they have not made any more firm decisions beyond that, in terms of how long the suspension will last. The collective bargaining agreement allows teams to suspend a player for up to four games for detrimental conduct.
That doesn't mean Timmons will miss all four games, a source told Darlington.
The Dolphins have been quiet on details about why Timmons left the team over the weekend. Sources told ESPN's Adam Schefter that Timmons was dealing with "a personal matter" and was scheduled to meet with doctors on Monday to try to figure out what is wrong.
Backup linebacker and special-teamer Chase Allen started in place of Timmons and had four tackles against the Chargers.
However, the most likely in-house replacement moving forward is veteran linebacker Rey Maualuga, who signed last month but reportedly joined the team overweight.
Maualuga, who was an eight-year starter for the Cincinnati Bengals, is close to working himself back into football shape.
In light of Timmons' suspension, the Dolphins acquired linebacker Stephone Anthony in a trade with the New Orleans Saints. Miami is sending a 2018 fifth-round pick to New Orleans for Anthony, ESPN's Adam Schefter reported. Anthony was a first-round pick for New Orleans in 2015; he has not played yet this season because of an ankle injury.
It already has been a trying start to the season for the Dolphins (1-0). They lost starting quarterback Ryan Tannehill (knee) for the year during the first week of training camp, and they recently were displaced for nine days in California because of Hurricane Irma.
Miami will travel to play the New York Jets (0-2) at MetLife Stadium on Sunday.
ESPN's Mike Triplett contributed to this report.