Ellis crushed the first offering he saw from former A's closer Huston Street down the left-field line.
Unfortunately, it was just a bit outside of fair territory.
Street rebounded to send Ellis down swinging, then got Adam Kennedy to fly to left as the A's fell in a comeback attempt for the second time in as many days, 3-1.
"He just missed," manager Bob Geren said. "If that lands, we could have won the game."
The A's hoped their seven-run effort over the final three innings in Saturday's 11-9 loss would give them some momentum against Colorado starter Aaron Cook.
Ten Oakland runners reached base against Cook, but the veteran righty was able to limit the damage. Cook went eight-plus innings, giving up only one run and striking out four to improve to 8-3 on the season.
"It's very frustrating," said Jack Cust. "You have to try to do a bit less. ...Don't try to kill the ball. That's easier said than done."
After surrendering two runs in a 37-pitch second, Vin Mazzaro settled down to hold the Rockies to one run in his final three-plus innings.
Mazzaro allowed five hits, walked four and struck out eight.
"They were able to put some good at-bats together against him," Geren said. "They fouled pitches off, got his pitch count up close to 110 pitches. ...He held a good offense down for the most part."
With Ellis in the lineup, Kennedy slid over to third base, making his first career start at the position.
Kennedy looked right at home on the hot corner, coming up with each of four defensive chances cleanly and narrowly missed throwing out Troy Tulowitzki at first base on a barehanded pickup.
Ellis and Nomar Garciaparra are now healthy, but that doesn't mean the A's are out of the woods on the injury front.
Josh Outman was transferred to the 60-day DL on Sunday because of his sprained left elbow.
He's scheduled to see Dr. James Andrews in Birmingham, Ala., on Monday.
"It's a big loss to the starting rotation," Mazzaro said. "We've got Gio [Gonzalez], though. He's a good, young lefty who can do good things for us."