Sweden's Isak was forced off at half-time during Sunday's 2-1 win at Wolverhampton Wanderers with what Howe thought was an eye issue, but it turned out he had a foot injury.
"It was a strange one with his eye. The ball grazed his eye, we thought that was the main reason for him coming off the pitch. Then we found he had a knock on his foot," Howe told reporters on Friday.
"We will find out more today and make a decision on him."
However, former AC Milan midfielder Tonali, who played two matches for Italy earlier this month, came off the bench on Sunday to impress Howe with his performance.
"I think Sandro is in a much better place, the international games really helped him. He’s certainly pushing to start but I want everyone pushing to start... we have really good competition for places in midfield," the manager said.
Newcastle, unbeaten with three wins in four Premier League matches, will also be without midfielder Lewis Miley, who is yet to return to full training after suffering a metatarsal fracture in July.
England striker Callum Wilson, out with a back injury, is looking to return to full training by next month's international break, Howe added.
