Either way, they will have to play a quarterback who hasn't appeared in a game this season.
Keenum started eight games for the Texans last year, losing all of them, before he was released in August when Ryan Mallett was acquired in a trade. He was on the St. Louis practice squad but signed with Houston after the Texans' loss on Sunday to Indianapolis included Ryan Fitzpatrick breaking his leg and rookie Tom Savage suffering a knee injury.
Lewis was signed on Nov. 26 after being released by the Bills in training camp. He went 2-3 in five starts for the Bills last year.
Coach Bill O'Brien said he's close to making a decision on a starter, but that he doesn't plan to announce his choice before Sunday.
''I'm going to keep that close to the vest,'' he said.
He did say that the Texans will use a game plan that is very different from what they've used previously.
Keenum, who set numerous NCAA records at the University of Houston, was glad to be back in Texas and to have the chance to compete for a job again. When asked about returning to Houston, the folksy Texas native, who speaks with a pronounced Texas lilt, partially quoted something his former coach Art Briles, currently at Baylor, used last week during a rant after the Bears were left out of the college football playoff.
''This is home for me,'' he said. ''I heard a coach say the other day that I really admire: `When I die they're going to bury me in Texas. They're not going to bury me in St. Louis.'''
He was asked if he expects to start on Sunday.
''I don't expect anything,'' he said. ''I'm expecting to go out there and have a good practice (Friday). I keep saying that but really that's all I can do right now. I can't look forward. Can't even look back. It's just about right now. It's a pretty special opportunity here today.''
Keenum was with the team when the offense was installed in training camp and hopes that will make the transition easier. He has a simple plan for the rest of his week.
''Just study,'' he said. ''Just live in the quarterback room watching film and just going over everything.''
O'Brien thought Keenum did a good job in his first day of practice on Wednesday.
''He comes right back into the fold and really has a good recall of what we were doing back when he was here,'' he said. ''That's impressive. Didn't take long to pick right back up.''
Keenum threw for 1,760 yards with nine touchdowns and six interceptions last season. He feels like he has some unfinished business here and hopes he gets another chance to play after losing each of his starts last year.
''It would mean a lot,'' he said. ''I want to win. I love to win and that was tough. To come out and have a chance to win a game would be pretty incredible.''
Lewis believes he has made a lot of progress since joining the team less than a month ago and is confident he'll be ready if he starts against the rugged Baltimore defense.
''It's a great opportunity,'' Lewis said. ''Sometimes (starts) are far and between and things don't always happen the way you want them to happen. You just got to make sure you're ready when that time presents itself.''
O'Brien has known Lewis since he was the offensive coordinator and quarterbacks coach at Duke when Lewis was a freshman.
''He's got really good poise,'' O'Brien said. ''He's a guy ... that nothing is too big for him.''
Lewis said Mallett and Savage have been around in meetings and the film room helping the newer quarterbacks prepare this week.
Receiver DeAndre Hopkins said they can't worry about the problems at quarterback and just have to do what they can to make it easier for whoever starts.
''With the leadership we have, guys have been through quarterback changes and you just have to go out there and do your job,'' Hopkins said. ''There's not uncertainty with us. It doesn't change anything how we prepare as a wide receiver group.''
AP NFL website: www.pro32.ap.org and http://twitter.com/AP-NFL