In Collins' place, associate head coach Michael Curry coached the team in a 111-95 loss to the Cavaliers.
"He had a spill this summer out in Arizona, and I think he got a concussion from it," said Collins' son, Chris, an assistant coach at Duke University. "And since then, he's had some dizzy spells."
Collins, 59, also has had some distinct lifestyle changes, including working 12-to-14-hour days with the Sixers.
"We didn't expect it; we didn't know it," Curry said of Collins' ailment. "You go up and down with your fatigue, but we didn't know he was having pain with his headaches and stuff like that."
Sixers president Rod Thorn said the team hopes that Collins' absence will be brief. No decision had been made on Collins' availability for Wednesday night's preseason finale against the New York Knicks at the Wells Fargo Center.
Thorn, along with Collins' staff, had no previous knowledge of the coach's condition.
"This is from a previous injury, with the headaches, according to him," Curry said. "They have to figure out why he's having those headaches, and hopefully he's OK.
"I told him, 'Good luck at the doctor's office, and we need you. We'll take care until you can get back.' "
Sixers forward Thaddeus Young said the team was told that "everything was going to be fine . . . and he's going to be back as soon as possible."
"They just came in and said he's flying back to take a few tests - that's about it," Young said. "They didn't tell us any more than that; they didn't want us to worry."
Thorn said he was told Collins had awakened in the middle of the night and felt ill.
Cavs 111, Sixers 95. The Sixers, led by Andre Iguodala with 19 points, trailed by 14 at halftime and by about the same number for much of the second half. For Cleveland, eight players scored in double figures, with forward J.J. Hickson powering his way to 22 points.
Starting five. On Tuesday, the Sixers changed their starting lineup to point guard Evan Turner, shooting guard Andre Iguodala, small forward Jason Kapono, power forward Elton Brand, and center Tony Battie.
Out. Sixers center Spencer Hawes missed his fourth consecutive game with a lower back strain.
Contact staff writer Kate Fagan at 856-779-3844 or firstname.lastname@example.org.