Brazil midfielder Guimaraes has been linked with a move to Paris Saint-Germain and Sweden striker Alexander Isak is reportedly a target for Arsenal and Tottenham.
Isak is on the shortlists for the Premier League player of the year and young player of the year awards after scoring 24 goals in all competitions so far this season.
Asked on Friday if he expects both players to be at Newcastle next season, Howe told reporters: "Yes, I do, I'm planning for them to be here. Yes, all my planning is with them in the squad."
Guimaraes, a £35 million ($44 million) arrival from Lyon, and Isak, who cost a club record £63 million from Real Sociedad, have been major success stories for Newcastle's recruitment team.
But Newcastle chief executive Darren Eales sparked speculation about a potential end of season sale when he revealed the club may have to cash in on their star players to comply with Premier League profitability and sustainability rules.
Howe warned that would set the team back significantly on the pitch after a difficult season that could still end with qualification for Europe.
Newcastle sit in sixth place in the Premier League with three games left, starting against Brighton on Saturday.
"We're looking to build a squad and a really successful team and to do that, we don't want to sell our best players," Howe said.
"That is hugely important for us. It would knock us back some considerable distance because we would have to start again."
Isak's prolific form this season, which includes 20 Premier League goals, will make him especially sought after by clubs across Europe.
But Howe is hopeful the 24-year-old is willing to extend his stay with the Magpies into a third season.
"We signed him knowing that he was young and we wanted to develop his talents and give him an opportunity to showcase what he could do," he said.
"For me, that was a long-term thing that we started with him. I think he's enjoyed the team that he's played in. I hope he does that for many years to come."
