In his first career start against the Nationals, Bettis went three innings in Denver on Aug. 16 before a two-hour rain delay prevented him from going any longer.
The Rockies won 6-2 and took two of three games in the series against the National League East leaders earlier this month.
Bettis is 10-7 with a 5.29 ERA this year and will make his 27th start of the season on Sunday, and that is tops among Colorado pitchers.
The Lubbock, Texas, native has pitched three times against Washington, including two outings from the bullpen in 2014.
It will be the last game of a road swing for the Rockies, who got second baseman DJ LeMahieu back in the starting lineup Saturday after he had missed three games with a left wrist injury.
"We missed some guys earlier in the road trip (because of injuries," said outfielder Charlie Blackmon, who had two home runs in a 9-4 win in 11 innings Saturday in Washington.
Bettis is 5-5 with a 5.21 ERA in road games this year and in his last start, on Aug. 22 at Milwaukee, he went five innings and gave up four runs (three runs) and four hits with a career-high seven walks and three strikeouts.
The Rockies are expected to face rookie right-hander Lucas Giolito, who made his first three starts of his big league career earlier this year for the Nationals.
"Tomorrow it's going to be Giolito, so I'm hoping he throws up a gem against a very tough lineup," Washington manager Dusty Baker said after Saturday's loss.
Giolito has been with Triple-A Syracuse the last few weeks and on Tuesday he was limited to one inning of work at Pawtucket, which seemed to indicate the Nationals would use him Sunday.
Washington announced Saturday that right-hander Joe Ross, who has been on the disabled list since July 3 because of right shoulder inflammation, would make a minor league rehab start Sunday for Syracuse against Pawtucket.
That is the spot in the rotation that Giolito held.
Giolito made two big league starts earlier this year against the New York Mets. He entered this season as the top pitching prospect in the Washington system and one of the best in all of the minors.
The Nationals bullpen has been taxed as of late, including extra innings Saturday.
"Well it's been tough," Baker said. "We knew it was going to be tough when we started out this streak. We didn't go very deep in that very first game which puts you behind the 8-ball.
"Boy that was a tough day, tough ending. We just have to come back and win the series tomorrow."
Another possible pitcher for the Nationals in the coming weeks is veteran right-hander Mat Latos, who was slated to start Saturday for Syracuse after he had been signed to a contract by Washington.
The Rockies have played well against Washington this year, with three wins in five games.
"They practice, too," Rockies manager Walt Weiss said.
Weiss said having Blackmon and LeMahieu at the top of the lineup makes a big difference.
"Both are getting on base at a really high rate," Weiss said. "Having those guys changes our offense. It has been a domino effect the last few weeks, missing some guys one after another. Guys have stepped up and played pretty well. It is nothing like having your horses in there."