Earl was hurt 15 minutes into Bolton's 2-0 defeat at Wycombe on Saturday.
He damaged his ankle making a tackle on the hosts' Paul Smyth and Bolton say he will be out between eight and 12 weeks.
Bolton also confirmed that a similar injury had been suffered by their striker Connor Hall, that believed to have happened in a recent friendly game.
Earl is expected to have treatment and under his rehabilitation at North End before resuming his loan spell with Wanderers.
PNE loaned the 20-year-old out in order to get a run of first-team games, with him having to play second fiddle in the main last season to Andrew Hughes.
Although the loan deal was agreed with Bolton last week, it was only confirmed when Bolton published the team sheet for the Wycombe game an hour before kick-off.
The paperwork had been registered on time but the move needed approval from the FA and EFL, hence the lateness of the announcement.
What is proving to be even more frustrating for Earl is that he was injured tackling a player who was then to be ruled offside.
Only after the tackle had gone in did the flag go up for offside.
Earl’s loan is to January 5 so even if he is out for the full 12 weeks he should be back to play in early November.
This is his second spell out on loan, having played for Lancaster City at the end of the 2016/17 season.
He broke into the PNE team early in the 2017/18 campaign when Greg Cunningham was injured and had a 12-game run in the team.
Having let Earl go on loan, it remains to be seen whether North End will bring in cover at left-back.
A bid for Derby’s Max Lowe was rejected last week and as things stand PNE won’t return with a fresh offer.
Earl is one of four players to have been loaned out by North End so far.
Chris Maxwell has joined Hibernian for the season and was on the bench for their SPL opener against St Mirren at the weekend.
Graham Burke is on a 12-month loan at Shamrock Rovers while first-year pro Jack Armer has been playing for Lancaster ahead of a three-month loan.