The Cowboys acquired Cassel from Buffalo on Tuesday, sending a fifth-round pick in 2017 to the Bills and getting their seventh-round pick that year.
Both teams announced the trade, and two people familiar with the deal told The Associated Press about the draft picks involved because the round of the picks weren't disclosed.
Brandon Weeden is expected to start Sunday at home against Arizona (2-0), but Cassel is a veteran with far more experience than the fourth-year pro.
Cassel has 71 starts in 90 games over 11 seasons, not counting the opener this year when he took the first snap for the Bills with Tyrod Taylor lined up at receiver.
The Cowboys traded for the 33-year-old Cassel a few hours after putting Romo on the injured list with a designation to return.
The first possible game Romo, who was injured in last weekend's 20-10 win at Philadelphia, is Nov. 22 at Miami. He can practice again in six weeks, starting Nov. 4.
The Bills acquired Cassel in a trade with Minnesota in March in what became their latest offseason quarterback shuffle.
He provided the Bills with veteran depth and was given a shot to compete for the starting job along with Taylor, a free-agent addition, and former starter EJ Manuel.
The three-way competition lasted until the final week of the preseason when Taylor was selected as the starter by coach Rex Ryan.
The Bills then made a calculated risk by releasing Cassel as part of a cost-cutting move because his $4.15 million base salary was deemed too expensive for a backup. Days later, the Bills re-signed Cassel to a one-year contract with a $2 million base salary.
Cassel said he turned down offers to sign with other teams to stay in Buffalo because he was familiar with the Bills' system and because he didn't want to relocate his wife, who was eight months pregnant.
Weeden, who turns 32 next month, has lost eight straight games as a starter, including last year against Arizona when Romo was out with a back injury. The skid goes back to his two-year stint in Cleveland, where he had a 5-15 record after the Browns drafted him late in the first round in 2012.
Before the trade, owner Jerry Jones said on his radio show Tuesday that the Cowboys were looking for a backup to Weeden, not someone to replace him while Romo is out. The Cowboys brought in several quarterbacks for a workout before deciding on a trade instead.
Weeden was 7 of 7 with a clinching 42-yard touchdown to Terrance Williams in last week's 20-10 win over the Eagles.
"Certainly we think Weeden has really progressed," Jones said. "I think we all feel good about what we got."
Kellen Moore, a fourth-year player who hasn't appeared in a regular-season game, is likely to get called up from the practice squad while the Cowboys get Cassel familiar with their system.
NOTES: The Cowboys signed RB Gus Johnson back to the practice squad after waiving him last week. Johnson takes the spot of LB Joe Thomas, who was signed by Green Bay on Monday.
AP Sports Writer John Wawrow in Buffalo, New York, contributed to this report.
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