Leinart was 10-of-14 for 177 yards and a score, all in the second quarter against what amounted to the Denver junior varsity.
Twenty-nine Broncos players sat out the game, as is the custom for the final preseason contest for coach Mike Shanahan.
Third-string quarterback Darrell Hackney threw for 208 yards, including an 87-yarder to Keary Colbert that put the Broncos up 21-14 with 8:51 remaining. Denver's Jack Williams picked off Anthony Morelli's pass and returned it 87 yards for a score with 5:42 to play.
Although the performance came against Denver's reserves, it was a welcome reversal for Leinart after an awful outing last week at Oakland put his status as starter in doubt.
His job seemed even more in jeopardy when Warner was named the starter against Denver.
Warner was 4-for-4 for 48 yards but couldn't get Arizona on the scoreboard.
Afterward, Whisenhunt praised the play of both quarterbacks and wouldn't commit to which would start in the Sept. 7 opener at San Francisco.
"We've got some tape to look at," Whisenhunt said. "We've got to go back and evaluate all the information that we've compiled over the last six months. ... It's going to be a tough decision."
Leinart entered the game on the last play of the first quarter and was promptly sacked. But after that he was 4-for-4 for 87 yards on a seven-play, 90-yard drive that culminated with his 17-yard touchdown pass to tight end Leonard Pope.
"We've said before a lot of times when Matt doesn't play well he seems to respond and play with a chip on his shoulder," Whisenhunt said, "and he did that tonight."
Leinart had left the locker room before reporters were allowed in, but Warner said he expected a decision from Whisenhunt soon.
"We knew there was going to be a competition," Warner said. "We knew there were going to be a lot of things involved and that it was going to have to play itself out."
Arrington ran up the middle on a 28-yard TD that put the Cardinals (2-2) ahead 14-10 with 1:28 to go in the half.
The Broncos (2-2) rested all of their defensive starters and all but two of their starters on offense. In addition, most of the top Denver reserves sat out the game in preparation for Sept. 8 opener at Oakland.
Still, Hackney and Anthony Aldridge had big nights. Aldridge, signed as an undrafted rookie free agent out of Houston, gained 110 yards on 23 carries.
"We got a chance to evaluate a lot of second- and third-team players," Shanahan said. "We had some great competition out there. We'll go back look at that film later on tonight, early in the morning and make some decisions tomorrow who our top 53 guys will be."
Arizona opens its regular season Sept. 7 at San Francisco.
Cardinals NT Alan Branch left the game with a sprained ankle in the second quarter. ... Arizona starting LT Mike Gandy had a slight knee sprain but returned to play another series. ... Green went out with a mild hamstring injury. Shanahan said he didn't know how serious it was.