Both players were out with injuries during the international break, with Kovacic not featuring since and Grealish appearing as a substitute in Saturday’s 2-0 victory over Nottingham Forest.
With the City boss set to make changes to his starting line-up for the third round clash against the Magpies, he confirmed that both will play at St James’ Park.
“Jack is much better, Mateo as well,” Guardiola said in his prematch news conference.
“I don’t know how many minutes, from the beginning or second half, but they’re going to play minutes, yeah.”
Guardiola is likely to make a number of changes with early season injuries meaning many of his squad have already played a lot of minutes.
“He will start,” the boss added.
“Not just this week but for many times since he arrived, Rodri has been really important, his consistency, he knows everything, the game like what happened against Forest.
“The second half was a little bit more defensively, you have to do the process, and [Phillips] made three or four incredible options to help us breathe.
“We have to make the process in small spaces and tried to help him to make this step.
“Of course the game tomorrow is important for the next ones, we’ll see in every single game and I will decide what is best for the team.”