Anthony Fenech of the Detroit Free Press reported Verlander cleared revocable trade waivers on Aug. 4, making him eligible to be traded to any team through the end of the month.
The Astros have been connected to Verlander since before the non-waiver trade deadline on July 31. Nick Cafardo of the Boston Globe reported on Aug. 19 that Houston hasn't "closed the door" on making a deal for the 34-year-old.
The addition of another suitor into the mix could make it easier for the Tigers to leverage offers for Verlander in an effort to get closer to what they deem to be fair value.
Verlander has two more guaranteed years left at $28 million per season on his contract with a vesting option for $22 million in 2020, per Spotrac.
After a slow start to the 2017 season, Verlander has a 3.90 ERA with 167 strikeouts in 166 innings. He has a 2.49 ERA in nine starts since the All-Star break.