Simmons was scheduled to make his NBA preseason debut for the Philadelphia 76ers against the Boston Celtics on Tuesday (Wednesday AEDT), but will likely be out for at least three months after suffering the injury in practice on Friday.
"It's like, 'damn'," James said, talking to reporters after Cleveland Cavaliers' practice on Monday.
"That's the first thing that came to my mind."
Although they play on opposing NBA teams, James, who led the Cavaliers to an NBA championship last season, has taken Simmons under his wing.
The Melbourne-born 20-year-old signed with James' Klutch Agency and spent part of the off-season working out with James and another All-Star, Dwyane Wade.
James said he heard Simmons would be out for "three-to-four months" but added he was a young guy and would be "fine".
"I've already done that and I'll continue to do it," James, when asked if he had reached out to Simmons, said.
"He's like a little brother of mine and I've been with him and counselled him for the last four-to-five years now.
"I'll continue that."
James said Simmons can't attempt to rush the recovery process and get frustrated about not playing.
"He has a long career in front of him and he just has to be patient, let the foot heal and when he gets back on the floor then he'll show why he's the number one pick."