Isak is training alone after expressing his desire to leave, despite having three years remaining on his contract with the Magpies.
Newcastle has already rejected a £110 million offer from Liverpool for the striker and is reportedly holding out for a British record fee of £150 million.
Meanwhile, the Newcastle hierarchy is working on bringing in new attacking talent, but an improved second offer of £55 million for Wolverhampton Wanderers forward Jørgen Strand Larsen has been turned down.
What Guimarães said
"I haven't had any contact with him. We train in the morning, and he's training at a different time when none of the team is around. It's a very difficult situation, and it's out of our control.
We want what's best for the club and the team. The board and the owners are handling this matter, and I hope they resolve it in the best way possible.
We definitely feel the absence of our top scorer from last season. We hope to get our best player back," Guimarães told ESPN Brasil, via Sportal.
Meanwhile, Eddie Howe has claimed that the club can not go a whole season without a striker following Isak's decision to leave the club.