The Houston Astros' scheduled starter for Wednesday night, Brad Peacock, is also set to return to the rotation after a stint in the minors.
After losing Lee to an injured left elbow, the Phillies (50-63) are expected to summon Buchanan from Triple-A Lehigh Valley before Wednesday's game. He was previously called up May 24 after Lee strained the same elbow, and went 5-5 with a 4.40 ERA in 10 starts for Philadelphia.
The right-hander seemed to get comfortable in his final five outings, going 3-2 with a 2.93 ERA.
"He made a good showing with us when he was here with his starts," manager Ryne Sandberg told MLB's official website. "He showed his composure out there. He gave us a chance to win when he pitched."
He'll make his first interleague start while pitching opposite Peacock (3-7, 4.93), who struggled mightily in the final two starts in his previous stint in the majors. The right-hander was pulled after recording one out in an 11-0 loss to Boston on July 13 and was charged with seven runs, seven hits and five walks in 3 2-3 innings of a 9-7 defeat in Oakland on July 23.
Houston (47-66) needed a new starter after trading Jarred Cosart to Miami in a six-player deal Thursday. Peacock is getting that call after working on the command of his fastball at Triple-A Oklahoma City.
"There were a couple of things (pitching coach Brent Strom) was able to look at the video and point out, and that was part of the reason we felt like we needed him to go to Oklahoma City," manager Bo Porter said. "We made the decision to send him to Oklahoma City, it all started with his ability to command the fastball."
Peacock will face a Phillies lineup that's scored two runs in its last 36 innings. Ryan Howard drove in both of those in a 2-1 victory in 15 innings in Tuesday's series opener.
The Philadelphia slugger, 1 for 25 in his previous seven games, hit a solo homer to snap his club's 22-inning scoreless streak and singled in Grady Sizemore for the winning run. Howard has started nine of the last 10 games after being benched for three straight July 23-25.
"You guys forget what I've done," he said. "People look at right now. I've hit balls hard over stretches. All you can do is hit balls hard and once it leaves the bat, it's out of your hands."
Howard has never faced Peacock, who won his only start against Philadelphia on Sept. 22, 2011, in his rookie season with Washington. The Phillies could have Domonic Brown back in the lineup for Wednesday.
Brown missed three straight games with an infection in his throat and was limited to a pinch-hit appearance Tuesday.
George Springer, meanwhile, is expected to miss another game but could return for Thursday's series finale. The rookie outfielder, who has 20 homers in 78 games, is rehabbing a strained left quad at Class A Quad Cities.