Withey averaged 2.6 points and 1.7 rebounds in 37 games for New Orleans last season.
The 23-year-old Valanciunas boasts a career .553 shooting percentage. He peaked last season at 57.2 percent and averaged a career-high 12.0 points a game while leading the team at 8.7 rebounds and 1.2 blocked shots.
Valenciunas also averaged 11.3 points and 9.3 rebounds in four playoff games.
"Moving forward, our sports performance staff will work with Joel to guide his return to play and monitor his progress against targeted benchmarks," 76ers president of basketball operations and general manager Sam Hinkie said. "Our team will work with Joel to create the best possible environment for healing so he can achieve his goals."
Embiid is expected to miss the entire 2015-16 season because of the foot issue. He also underwent bone graft surgery in May.
The Mavericks, though, are confident McGee will be healthy for the start of training camp.
The deal is for two years at the league minimum, with a team option for the second year. McGee played in just 23 games last year for the Nuggets and 76ers before being waived in March. He has only played 28 games over the past two seasons.
In seven NBA seasons, McGee is averaging 8.4 points and 5.5 rebounds.
Brand, who appeared in 36 games last season for the Hawks, is a former All-Star who doesn't have much left to accomplish on the basketball court.
"I could get in shape if I got the call but this looks like the end of the run for me, right now it is family time,” Brand said.
McGee averaged 4.6 points and 2.7 rebounds in 23 games for the Nuggets and 76ers last season.
Villanueva managed 6.3 points and 2.3 rebounds a game as a reserve for Dallas last season. Dalembert averaged 4.0 points and 5.3 rebounds for the Knicks a year ago.
Haywood averaged 1.6 points and 1.3 rebounds in 22 games for the Cavaliers last season.
The trade was first reported on July 7, but the Celtics needed to make various roster moves in order to facilitate the trade.
"On behalf of the entire Golden State Warriors organization, we thank David Lee for his contributions both on and off the court over the last five years," Warriors GM Bob Myers said.
Lee averaged 7.9 points and 5.2 rebounds last season in a reduced role, while Wallace was a non-factor for the Celtics, scoring just 1.1 points in nine minutes.