Morris, 34, played 11 years in Buffalo, Miami and New England but has not played this year after getting cut by the Patriots in training camp. He had a workout with Kansas City this season.
"I hadn't heard anything for a while so I assumed that I'm going to ride into the sunset," Morris said. "So I got the phone call. ... I all but said I was done in my mind, but it didn't turn out that way."
To make room for Morris, the ex-Texas Tech running back, the Cowboys placed rookie running back DeMarco Murray on injured reserve because of an ankle injury suffered Sunday against the New York Giants that requires surgery.
With Felix Jones as the only healthy tailback on the roster, the Cowboys needed Morris. He has rushed for 2,955 yards in his career but could move into a third-down role to help spell Jones, who had 106 yards on the ground and six catches for 31 yards against the Giants.
"He's certainly the right kind of guy in the role that he's had in his career," coach Jason Garrett said of Morris. "He's done it as well as anybody."
Addressing another injury, the Cowboys also signed safety Mana Silva off Buffalo's practice squad since Barry Church is heading to injured reserve because of a shoulder injury that will require surgery.
Silva, 6-foot, 206 pounds, went to training camp with Baltimore as an undrafted free agent out of Hawaii and spent part of the season on the Ravens' practice squad. He had seven tackles in the preseason. At Hawaii, he intercepted 14 passes.
The Cowboys needed to sign Silva after Church's right shoulder popped out twice in Sunday's loss to the New York Giants. Church was replaced by Danny McCray, who did not practice last week because of a high left ankle sprain.
Center Phil Costa, who suffered a concussion in Sunday's game against the Giants, did not take part in Tuesday's walkthrough practice. While owner Jerry Jones expressed optimism that Costa would be able to play vs. the Bucs, the second-year center has to pass a series of mandated tests before being allowed to practice.
If Costa can't play, undrafted rookie Kevin Kowalski would start.
Todd Archer covers the Cowboys for ESPNDallas.com.