Former first-round pick Jiovanni Mier, a shortstop who was hitting .309 with two homers and 13 RBIs in 26 at-bats at Class A Lexington, is expected to miss about three to four weeks with a hamstring injury, Astros director of player development Fred Nelson said.

Mier is going to be examined in Houston in the next few days.

"We just don't know [the severity] until he's evaluated and we get a clear picture of how severe it was," Nelson said. "It's a pretty significant hamstring injury."

In other injury news, right-hander Jack Armstrong, whom the Astros selected in the third round out of Vanderbilt in last year's Draft, is still wearing a brace after undergoing Tommy John surgery two weeks ago. Nelson said team doctors are pleased with his progress, but he'll miss all of 2012.

Foltynewicz enjoying success with Lexington

HOUSTON -- Mike Foltynewicz is admittedly more confident and comfortable in his surroundings this year, and now the right-hander is putting up some impressive numbers at Class A Lexington.

Foltynewicz, who was the No. 19 overall pick in the 2010 Draft out of a Minnesota high school, entered Monday's start at Asheville with a 4-1 record and 1.62 ERA in six starts for the Legends. He's allowed 27 hits and 12 walks while striking out 29 in 33 1/3 innings.

"Everything's gone pretty smooth so far, a lot better than last year," Foltynewicz said. "I'm more confident than last year in knowing what to expect. I'm getting more strikeouts than last year, more ground balls, and I'm throwing strikes and getting outs. I'm getting the ball down the strike zone and the defense has been making plays."

Foltynewicz, 20, is in Lexington for the second year in a row. His first full season in professional ball in 2010 was a rough one. He struggled for much of the year with inconsistency, as he tried to adjust to the grind of pitching every fifth day.

"I think last year I was unprepared a little bit," he said. "I didn't know what to expect, but I got better prepared and worked on a lot of little things in instructional league, as well, and came to Spring Training ready to go for a full season. I'm more confident and ready to go at batters."

Working closely with pitching coach Dave Borkowski, Foltynewicz has become more mentally prepared in the minutes leading up to his starts and improved his focus.

"Sometimes, young kids try to do too much and get away from the game plan," Astros director of player development Fred Nelson said. "He's settled in, he's pitching with confidence. He's had success, and I think we're reaping the benefits of it right now. We expect him to go out there and string together a good number of quality starts over a period of time."

Foltynewicz throws a two-seam fastball and pitches around 92-94 mph, though he touches 96, and occasionally 97. He's been getting a lot of swings and misses on his changeup, which is about 8-10 mph slower than his fastball and is his second-best pitch. He's tightened up his curveball and is throwing it harder.

"I'm having a pretty good year so far, and I just want to keep that in the back of my mind and stay confident and see where it leads me," Foltynewicz said.

Nelson checks out extended spring camp

Astros director of player development Fred Nelson is spending this week at extended spring camp, which takes place at the club's facility in Kissimmee, Fla. There are currently about 70 players participating, ranging from those on medical rehabs to younger players who will be sent out when the short-season clubs begin play next month.

The players participated in a six-inning simulated game Monday before playing the Tigers at Osceola County Stadium. When the Astros play the nearby Nationals, they will play a doubleheader consisting of seven-inning games.

"It gives our pitchers more work and preparation for the short-season clubs starting out," Nelson said.

The healthy players are used to fill out the rosters at rookie level Greeneville, short-season Tri-City and the rookie Gulf Coast League Astros, along with those players taken in June's First-Year Player Draft. The three short-season clubs carry a total of 90 players.

"We have the Draft to help us supply those players, but you don't sign everybody in the Draft, and there will be some guys who will be released after the Draft that aren't going to make clubs and we feel we've seen enough of them," Nelson said.

Among the players who have been sent back to extended spring camp after early-season struggles in Lexington are pitchers Tanner Bushue, a right-hander, and Kyle Hallock, a left-hander.

Among the players rehabbing in Kissimmee are Taiwanese right-hander Chia-Jen Lo, who had Tommy John surgery last August and could pitch in a game by year's end, and former second-round pick Vincent Velasquez, a pitcher who had Tommy John surgery in 2010 and will be sent out to a short-season club next month.

Worth noting

• Class A Lancaster outfielder Adam Bailey is on a tear, going 10-for-19 with four homers and nine RBIs in the last five games he's started. He hit two homers in consecutive games on Friday and Saturday, driving in seven runs, and he entered Monday hitting .306 with six homers and 15 RBIs. "He's a pretty good left-handed hitter who's put himself on the radar screen," Astros director of player development Fred Nelson said.

• Right-hander Tyler Perez, a 17th-round pick out of Fresno City College last year, came out of extended spring camp and allowed two hits and one run in six innings in his season debut at Lancaster on Sunday. He struck out four and didn't walk a batter. He was 2-5 with a 5.07 ERA in 13 starts with rookie level Greeneville last year.

• Right-hander Nick Tropeano, Houston's No. 17 prospect, struck out eight batters in seven scoreless innings in Lexington's win over Asheville on Saturday. Tropeano's 1.87 ERA through six outings ranks fifth among active South Atlantic League starters.

• Top pitching prospect Jarred Cosart is scheduled to make one start for Triple-A Oklahoma City on Tuesday before being sent back to Double-A Corpus Christi. That spot in the rotation originally belonged to Aneury Rodriguez, who will instead pitch in Houston on Tuesday.