Hyndman, 20, hasn’t trained since the first day of the American tour while Benik Afobe has also picked up a knee injury, which will be scanned when the team return from the States today.
"Emerson Hyndman hasn't trained since the first day of the tour," Howe told Cherries Player HD.
"It's a mysterious one because he doesn't remember any tackle, any knock or any twist but has ended up with an ankle injury, so he will probably be another couple of weeks. He might miss most of pre-season.
"Benik just took a knock to the knee in training so we are not sure of the extent of his injury yet, but we are going to have him scanned when we get back.
"We had a whole host of players missing from the Minnesota game through little niggles really. We just didn't want to push them too far so it was very much a case of being conservative with them."
The Cherries kicked off their pre-season with a comfortable 4-0 win over Minnesota United. Click here to read the match report from America.