The 64-year-old Majerus, who coached at the University of Utah from 1989-2004, is hospitalized in California undergoing evaluation and treatment for an ongoing heart condition, school spokesman Brian Kunderman said.
"His health is the most important issue right now, and our hope is for a full and complete recovery," athletic director Chris May said in a statement.
The school said administrators have spoken with Majerus' physicians about his condition, but could not provide any additional medical information. Majerus' cell phone was not accepting messages.
Majerus has a history of heart surgeries dating to 1989, and is on blood thinner medication. He had a stent inserted in his heart last August in Salt Lake City, and missed a handful of games in the 2011-12 season after gashing his leg in a collision with players who had been diving for a loose ball.
Saint Louis was 24-8 last season, Majerus' fifth with the school, and made its first NCAA tournament appearance since 2000. The Billilkens, a typical blend of area, national and international players with key contributors from Australia and New Zealand, advanced to the third round.
Majerus is 95-69 in five seasons at Saint Louis and in 25 seasons he has a career record of 517-216, among the best winning percentages for active coaches, with a dozen NCAA appearances. Saint Louis' NCAA appearance this spring was a 14-win improvement from the previous season.
Saint Louis' victory over Memphis in the NCAA tournament was the school's first since 1998.
Majerus came out of retirement to take on the rebuilding job at Saint Louis in 2007. He left Utah in 2004 after experiencing chest pains, and cited health issues when he backed out of the Southern California job three days after accepting the position.
Crews joined the coaching staff last fall. He has 24 years of head coaching experience, with seven seasons at Army from 2002-09 and 17 at Evansville prior to that. Crews has a career record of 354-348 as a head coach.
"Jim Crews is a long-time professional and is all-in with the task of leading the program this year," May said. "He has led teams to numerous NCAA tournaments, which is our goal. We welcome and encourage the Billiken faithful and the community to rally around this group of young men as we continue to prepare for what promises to be an exciting basketball season."
Evansville had four NCAA tournament teams under Crews, who was a member of two NCAA championship teams at Indiana including the 1976 that finished 32-0 the last unbeaten record for a Division I school.
Crews was an assistant under Bob Knight for eight seasons, including another national title in 1981.
"Coach Majerus is not only a colleague, but a dear friend, so this is a difficult time," Crews said. "We are fortunate to have a group of outstanding young men.
"This past year, they have been a delight to work with, and our staff is excited to serve them. Hopefully, our fans will rally behind this team and enjoy a great season."