Brazil midfielder Guimaraes is reportedly the subject of a £70 million offer from Arsenal.
Guimaraes is currently on holiday after Brazil's World Cup last 16 defeat against Norway earlier in July.
The 28-year-old is set to return to Newcastle on Friday ahead of the club's pre-season training camp in Spain.
But Arsenal's mooted interest in Guimaraes is providing a headache for Howe, who lost Alexander Isak to Liverpool after a protracted transfer saga last year.
Speaking after Newcastle's 1-1 draw in a pre-season friendly at Gateshead, Howe said: "We don't want to lose our best players
"I've spoken to him (Guimaraes) as I would do. Bruno is just a fantastic person. We've had some really good conversations, both before the World Cup, during the World Cup and after the World Cup.
"I don't know what's going to happen with Bruno's future. That's for other people to, one, speculate about, but also conversations that I'm not part of."
Isak effectively went on strike, refusing to take part in Newcastle's pre-season preparations 12 months ago, in a bid to force through his move to Anfield.
Arsenal are keen on Guimaraes after opting not to match Chelsea's bid in their British record £117 million swoop for England forward Morgan Rogers.
But Howe is hopeful Newcastle captain Guimaraes will opt to stay at St James' Park.
"There's nothing to say Bruno won't be back training with us. You've got to look at it in its own right," he said.
"Bruno is the captain of our football club. He's been an unbelievable player and person all through the time we have worked together.
"I speak on behalf of everyone at the football club. We absolutely love him and, of course, everyone will want him to stay."