According to Jay King of The Athletic, the team said Smart was experiencing flu-like symptoms, which resulted in him receiving an IV at New England Baptist Hospital.
Per ESPN's Adrian Wojnarowski, the restricted free agent signed a four-year, $52 million deal to remain with the Celtics.
After re-signing, Smart made it clear he was happy to be back in Boston, according to ESPN.com's Chris Forsberg: "This is where I want to be, and I'm ready to put a green jersey back on and get to work. I'm determined to help my teammates bring another championship to the best fans in the world."
The 24-year-old Smart appeared in 54 regular-season games in 2017-18, averaging 10.2 points, 4.8 assists, 3.5 rebounds and 1.3 steals.
The 2014 No. 6 overall pick will continue to be part of a young Celtics core that appears primed to compete for championships for many years to come.