Clark was ranked by recruiting analysts as the top high school tight end in the nation last season while at Avon (Conn.) Old Farms Prep.
"This is a trickier deal," Mora said. "It's more than just him feeling energy. They have to make sure his spleen is OK."
Mora said Clark would not redshirt this season.
"He'll play versus Virginia," Mora said of the Bruins' season opener on Sept. 5. "We didn't bring him here to redshirt, I can tell you that."
Clark caught 33 passes for 417 yards and six touchdowns at Old Farms last season. He is listed at 6-foot-6 and 247 pounds, but has lost considerable weight while he has been sick.