Isak made a transfer request a fortnight ago amid interest from Liverpool, which have since had a £110m offer for the Swede rejected.
Isak spent last week away from the Toon and instead trained in Spain using former club Real Sociedad's facilities.
Asked if the striker will be joining the squad back on Tyneside after the Asian tour, Howe told the Daily Mail: "You have to earn the right to train with us.
"We are Newcastle United. The player has a responsibility here to be part of a team and part of a squad - you have to act in the right way. So that is also at play here.
"We will make sure that any player does that to earn the right to train with the group. No player can expect to act poorly and train with the group as normal."
Howe added, "Of course I'd like him to be (there), but whether he will I don’t know at this moment in time. We have been here with the time difference, preparing for training and for games. Other people have been dealing with that situation back at home."