Ibrahimovic tested positive for COVID-19 on 24 September, three days after he had scored a Serie A double in the 3-0 win over Bologna.
Milan confirmed last week the striker is clear of the virus after having two negative tests, allowing him to leave a period of self-isolation that had caused him to miss four matches.
That put the 39-year-old in contention to play at least some part in Sunday's (AEDT) crunch Serie A clash, with the two Milan clubs looking as good as they have done for many years.
"Zlatan has been smiling and is in a positive and determined frame of mind," Pioli said as Milan looks to win its opening four league games for the first time since 1995, when Fabio Capello was coach.
"He has only been training for a week but is feeling fine. He had a problem, but I have found him to be the same as always.
"He is ready to play, but we still need to assess how many minutes he has got in his legs.
"Our start to the season with three league wins and going through in Europe was important and what I hoped for - now Zlatan's return is an additional boost going into the derby."
Milan is unbeaten since March – a run of 19 games in all competitions – while Inter is two points behind after finishing second and reaching the UEFA Europa League final in Antonio Conte's first season.
Except for Atalanta (56), Milan (54) and Inter (47) are the two teams who have gained the most points in Serie A in 2020.
Pioli added: "I am expecting a spectacular game. We will be facing a side with a system that sees players occupy precise positions on the pitch.
"Inter is very strong, but, like all teams, also has weaknesses. We need to remain composed and understand when it is necessary to press and when we need to bide our time.
"We will be intense and will not hold off. This will also apply to them but we have our own identity and way of playing out on the pitch.
"For us, the tests never stop: we know that this will be an important and difficult test, but we have prepared well in the little time we had available and feel ready."
Inter only finished one point behind champion Juventus last season, with Conte now looking to become the first Inter coach to win their first three Milan derbies.
Pioli added: "Unfortunately, there won’t be a crowd, but we’ll take to the pitch aware of the importance of this match and what it means. We feel the rivalry and it will be a heartfelt match.
"Our journey began a year ago, but I can also say that we’re still in the early stages. We still have plenty of room for improvement and will be facing the side everyone is tipping to win the title.
"But we are going to take to the pitch with the objective to be better than them. They are doing well, so beating such a strong side would give us enthusiasm and even more confidence in the qualities we have."