Maxwell, a big right-handed hitter, is expected to join the Royals for Thursday afternoon's game against Minnesota and be in the starting lineup against Twins left-hander Scott Diamond.
"We think he's going to be a strong contributor for us, especially against left-handed pitchers. He plays all three outfield positions well," general manager Dayton Moore said.
A corresponding move to make room on the 25-man roster will be announced before Thursday's game. Maxwell was with the Astros in Baltimore, about 45 minutes from his hometown of Olney, Md.
"I was definitely surprised, but I'm excited," Maxwell said. "I don't know much about Kansas City. I think the team is in the [American League] Wild Card hunt. That's what I heard at least. I'll be excited to join my new teammates and meet everybody. I definitely enjoyed my time in Houston. It was my first extended experience in the big leagues and I had a lot of fun this year and last year, and met a lot of great guys this year. I just wish them all the best."
Prospects acquired by Astros
Kyle Smith, rhp: Smith was more polished than most high school pitchers when the Royals selected him in the fourth round of the 2011 First-Year Player Draft. Accordingly, he has quickly moved through the Minor Leagues. At the time of the trade, Smith was in Class A Advanced Wilmington, where his 2.85 ERA ranked fourth in the Carolina League. Listed at 6-foot, 170 pounds, Smith is slightly undersized and doesn't blow hitters away. But he makes up for it with advanced pitchability and good secondary stuff. Smith's fastball sits in the low-90s and he mixes it with a solid curveball and changeup. He commands all three pitches well thanks to his smooth, easy delivery. Smith profiles as a middle of the rotation starter.
Maxwell has played just 40 games this season, hitting .241 with two homers and eight RBIs, as he battled a number of afflictions. He missed nearly two months and 51 games with a broken bone in his left hand after being hit by a pitch on April 23 by Seattle's Hisashi Iwakuma. While rehabbing, he was bothered by stomach ailments and food poisoning. Within a week after returning to the Astros' lineup, he dove for a ball on June 25 against St. Louis and sustained a mild concussion.
"I missed two months with my hand," he said. "The year definitely didn't go the way I wanted, but they always say try to finish strong, so that's what I'm looking forward to doing in the future."
A strong point for the right-handed Maxwell is that he has done well against left-handed pitching this year -- .302 (13-for-43), although his career mark against lefties is .253. Against right-handers, he's hit .203.
Another plus is his fielding ability. The 6-foot-5, 220-pounder was named the Astros' best defensive player in 2012 by Wilson Sporting Goods.
Maxwell showed power in 2012 when he banged 18 home runs and had 53 RBIs along with a .229 average in 124 games for the Astros. He has speed, evidenced by 24 stolen bases in 31 attempts in his career.
His addition gives the Royals five outfielders as he joins Alex Gordon, Lorenzo Cain, David Lough and Jarrod Dyson. Manager Ned Yost said his use of Maxwell in the mix will be a day-to-day proposition.
"Just play him if there's a good matchup for him, especially against left-handers," Yost said. "With one right-handed bat in the outfield [Cain], it's hard to give Alex days off. You can give Cain and Lough days off, depending, but there's just not enough right-handed bats to give Alex a day off. And with just one day off in the next 43 days, I'm going to have to find him a day or two hopefully. It just provides outfield depth, it gives us another bat off the bench, it gives us another option against left-handed pitchers."
Selected by Washington in the fourth round of the 2005 First-Year Player Draft, Maxwell played parts of three seasons with the Nationals. Traded to the Yankees in 2011, he was claimed off waivers by the Astros in 2012. He's not a pricey addition; his 2013 salary is $492,500.
"The other part of the deal is we have a player that we can control in the future as well," Moore said.
Smith, 20, was a fourth-round choice by the Royals in the 2011 Draft. From Lantana, Fla., he passed up a chance to play at the University of Florida to sign with the Royals.
A right-hander, Smith had a 5-4 record and a 2.85 ERA in 19 starts for Class A Wilmington. Last year, he had a combined record of 5-3 and a 2.86 ERA for Rookie Idaho Falls and Class A Kane County. He was ranked 11th by MLB.com in the Royals Top 20 Prospects.
"Kyle is a terrific kid and he's done an outstanding job and he's going to progress well, and he's going to be a Major League pitcher someday," Moore said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.