"My mother, Lavoyda, unexpectedly passed away Monday at her home in Texas," Urlacher said in a statement. "My first priority right now is to be with my family as we mourn her loss and make the arrangements to lay her to rest. This is a very difficult and emotional time for us and I sincerely ask that you respect our privacy. Thank you."
Lenard was 51. Lt. Brent Smith of the Titus County (Texas) Sheriff's Office said there was no evidence of foul play.
"Our thoughts and prayers going out to Brian and his family" read a tweet from Bears quarterback Jay Cutler.
Urlacher's status for Sunday's game in New Orleans against the Saints is uncertain.
"It was out of season when I went through it, so I can't tell you what Brian is going through right now," said coach Lovie Smith, who lost his mother Mae earlier this year. "But I know sometimes when you're going through tough times, the best thing for you to do is get back to something you like doing, and football is very important to Brian."
Urlacher's teammates tempered their concern about the impact on the team with their thoughts about their teammate and friend.
"That's going to be tough for our football team (if Urlacher doesn't play Sunday), but we're understanding," Bears receiver Roy Williams said. "Whoever takes his spot (if necessary) will play well.
"My thoughts and prayers go out to him and his family. He is the emotional leader of the football team, so we have his back."
Urlacher has played all 13 seasons in Chicago and is one of the more popular players in the Bears' locker room.
"It's hard, you feel for him," said linebacker Nick Roach, who is an option to slide over from strong side to replace Urlacher in the middle on Sunday if necessary. "Beyond what type of player he is, he's everybody's friend in the locker room.
"One of the greatest guys you can probably meet in this sport. It's tough to hear he's going through something like that."
Roach started three games for the Bears at middle linebacker in 2009. Reserve Brian Iwuh, who also has starting experience in the middle from his days in Jacksonville, could then be plugged in on the strong side. But Roach made it clear the Bears are not ready to rule out Urlacher this early in the week.
"We're just hopeful that he (Urlacher) will be back, but we'll have a plan if he doesn't make it," Roach said. "We'll all be ready."
Defensive end Israel Idonije focused on the personal impact on Urlacher.
"Our hearts are with him," Idonije said. "It's one of those times that you can't imagine, and 100 percent of our support is with him.
"At the end of the day, family and life comes first."
Cutler said Wednesday afternoon that it would be impossible to replace Urlacher if the linebacker is unable to play Sunday.
Since 2008, the Bears are 21-13 with Urlacher in the lineup and 7-8 without him.
"You can't replace a guy like him," Cutler said. "He's the heart and soul of this team."
ESPNChicago.com's Jeff Dickerson and Michael C. Wright contributed to this report.