HOUSTON -- Astros pitcher Bud Norris, who was placed on the 15-day disabled list Sunday with a sprained left knee, threw in the bullpen prior to Monday's game and was encouraged by how much better he felt. Norris could be sent on a rehab assignment by the end of the week.

Houston pitching coach Doug Brocail watched Norris closely and was pleased with what he saw. "Bud looks a lot better," Brocail said. "He said he was pain-free, so we're going to attack it that way. He'll probably have a day off, throw another light side. If he looks as good as he did today, we'll send him out on a rehab."

Norris, who is 5-4 with a 4.81 ERA in 13 starts, sprained his knee pitching against the Giants on Tuesday in San Francisco. He was scheduled to start Sunday against the Rangers, but the Astros called up left-hander Dallas Keuchel from Triple-A Oklahoma City to make his Major League debut.

Norris, 27, injured his hip flexor May 31 against the Rockies and wasn't the same. He was 5-1 with a 3.34 ERA in his first 10 starts of the season, but just 0-3 with a 13.09 ERA in his next three starts as he dealt with leg problems.

Martinez to undergo further concussion tests

HOUSTON -- The Astros ran outfielder Fernando Martinez through more tests on Monday to gauge his progress from a concussion suffered when he was hit in the head while at Triple-A Oklahoma City. He aggravated the condition at some point diving for a ball with the Astros.

Martinez was placed on the seven-day disabled list, which is new this year for players with concussions. He was played on the DL on June 9.

"He's still doing some tests," Astros manager Brad Mills said. "They want to run him through some drills on the field today, looking up at the lights and looking around to see how he's feeling. If that goes well, he's going to run through some concussion test to see how he's doing. Once he does that, we'll run him through more hitting drills and send him out on rehab."

Martinez, claimed off waivers from the Mets in the offseason, was hitting .319 with eight homers and 38 RBIs when the Astros called him up earlier this month. He appeared in only four games for Houston before going on the DL, notching one hit in 15 at-bats with nine strikeouts.

Schafer sits out opener; Maxwell starts in center

HOUSTON -- With the Astros facing a left-handed starter on Monday in Kansas City's Jonathan Sanchez, manager Brad Mills had Justin Maxwell in the lineup in center field instead of the left-handed hitting Jordan Schafer. Schafer is hitting .118 against southpaws this year.

Maxwell walked in the first inning and came around to score the Astros' second run when Sanchez's pickoff attempt at first went up the right-field line.

In the last 13 games, Maxwell has started seven games in center field and Schafer has started six, including all three games against the Rangers last weekend in Arlington. Schafer just recently got over an illness that cost him some playing time.

"It's been a tough last month really for him with the sickness and everything else," Mills said. "I know I was asked last week if it was a platoon thing, and I think that in the short-term it might be. Do I like platoon situations? No, I don't. Am I trying to make him a platoon player? No, I'm not.

"We have a long way to go yet in the season, and I think with the way things are working out, I think I'd like to give Maxwell some swings against lefties and keep working with Schafer and get him back in there and get him at-bats against lefties at times. Hopefully that can be the case here in the very short term."

Mills said he's explained the playing time to Schafer and told him not to panic. Mills said the Astros will face a lot of right-handers later in their current homestand, so Schafer should get plenty of starts upcoming.

"So it's not an issue," he said. "We'll get him one or two at-bats against a reliever or whatever in certain situations, getting him back in the flow of things instead of getting him two and three looks against a starter. I think that's probably the best thing. He's going to be OK. He skill set hasn't changed. We're still very excited about him. We love this guy."