Wanted man: Alexander Isak
Isak was at the Newcastle training ground on Monday after returning from Spain, as he waits on a second bid for Liverpool after their £110million offer was rejected.
Howe and the rest of the Newcastle squad had the day off after landing back from South Korea on Monday morning.
Isak missed the tour of Asia and has been receiving treatment for a calf injury.
Howe has said that striker Isak must "earn the right" to train with his squad.
“You have to earn the right to train with us,” said Howe.
“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.”
Newcastle have been linked with a move for RB Leipzig forward Benjamin Sesko, who is also wanted by Manchester United.
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It has been a challenging summer for Newcastle and former striker Alan Shearer has called their summer transfer window so far a "mess".
While adding Nottingham Forest winger Anthony Elanga and loan goalkeeper Aaron Ramsdale, they have offloaded Sean Longstaff and striker Callum Wilson.