The Indians optioned Brantley to Triple-A Columbus on Tuesday, the second time they have done so this season.
Brantley, who was batting .235 (8-for-34) since the All-Star break, earned a spot on Cleveland's Opening Day roster before rejoining the Tribe on July 4, following Shin-Soo Choo's trip to the 15-day disabled list.
But Choo's return last week has created a surplus of outfielders at manager Manny Acta's disposal, with Trevor Crowe and Austin Kearns also in the mix.
Acta saw improvement from the 23-year-old Brantley in recent weeks.
"I thought he made more solid contact, had a better swing, worked the count and played very good defense," Acta said. "This decision is not based on performance. We're just loaded with outfielders.
"Michael will be back soon. He's done just about everything he can do in the Minor Leagues."
To make room for Tomlin on the 40-man roster, Cleveland designated Columbus infielder Wes Hodges for assignment.