"Dakota wouldn't even know her if she saw a picture of her," a source told Radar of Rounds.
"Dakota will be shielded from the news" of Rounds' death, the insider added.
Rounds died on Monday from an apparent suicide, according to reports.
O'Donnell, 55, told US Weekly in a statement, "I am saddened to hear about this terrible tragedy. Mental illness is a very serious issue affecting many families. My thoughts and prayers go out to Michelle's family, her wife and their child."
Rounds had moved in to another marriage and child.
But the source told Radar, "Michelle suffered from a lot of things, not just depression. Those close to her are suggesting that drinking and drugs were an issue."
O'Donnell started dating Rounds in 2011 and they married in 2012.
They adopted daughter Dakota O'Donnell together but couldn't make the relationship work and went through a nasty divorce in 2015. They fought over custody of the girl.
But O'Donnell was eventually awarded full custody of Dakota, who was just 2 at the time.
As Radar previously reported, Rounds attempted suicide in September 2015, amid the fight for custody.
Officers were forced to break down the door to Rounds' room, sources told Radar back then.
"She had apparently taken some pills and drank wine," an insider revealed. "It looked to be a suicide attempt."
Rounds was taken to hospital in 2015 and recovered, but sadly, this week, she passed away.
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