CHICAGO -- Catcher Jason Castro was back in the lineup for Monday's series opener against the Cubs at Wrigley Field after being activated from the disabled list.

Castro missed 31 games because of swelling and excess fluid in his surgically repaired right knee.

"I'm excited to get back," said Castro, who hit .301 with two homers and 12 RBIs in 83 at-bats before going on the DL. "I felt really comfortable and just relaxed at the plate, and hopefully I can keep that up here and kind of pick up where I left off before [going on] the DL and even during my rehab assignment."

Castro combined to hit .556 with three doubles, a home run and two RBIs in seven rehab games between Double-A Corpus Christi and Triple-A Oklahoma City. More important, he caught back-to-back nine-inning games on Friday and Saturday.

"The ultimate goal was to get to where I was comfortable catching back-to-back nine and being able to do it comfortably," he said. "It felt good, and it went as planned and as hoped, so I'm excited to be back."

Manager Brad Mills plans to alternate between Castro and Chris Snyder every other day for the next week.

"We hope that's enough so when we do need to play [Castro] back to back, he's played enough and built up to that," Mills said.

Mills has 'heavy heart' following Pesky's passing

HOUSTON -- When manager Brad Mills found out that Red Sox legend Johnny Pesky had died on Monday at the age of 92, he sent a text message to his wife, Ronda, and told her he had a "heavy heart."

Mills spent six years as the bench coach of the Red Sox prior to being hired as Astros manager in 2010 and called Pesky a friend. Pesky spent the majority of his baseball career in Boston and remained a fixture at Fenway Park long after his playing days.

"The six years I was there, whether it was Spring Training or during the season, you would always sit down and visit with Johnny, because he's always got some stories to tell you about the game," Mills said. "He had a lot of knowledge about the game for playing the field and his time on the infield with Ted [Williams] and Babe [Ruth]. It's sad, but at the same time, it brings back a lot of reflection."

Pesky broke into the big leagues with the Red Sox in 1942 and batted .331 with 205 hits, finishing third in American League MVP voting. He missed three seasons due to military service during World War II and came back to record 208 and 207 hits in 1946 and 1947, respectively, and was named to his only All-Star Game in 1946.

He finished his 10-year Major League playing career as a .307 hitter with 404 RBIs, 867 runs and 1,455 hits. He managed the Red Sox from 1963 to 1964 and in 1980, and was associated with the franchise in numerous other capacities. His No. 6 is retired.

"You'd see him just about every game, in the clubhouse and dugout, and you'd sit down and chat with him," Mills said. "It was kind of cool."