At Monza, Sauber Junior Team ran with just one car, with Callum Ilott taking his first pole position of the season in qualifying.
Isaakyan has been racing cars since 2013, coming second in the 2017 World Series Formula V8 3.5 championship. He raced four rounds of the World Endurance championship last year with SMP Racing's LMP1 squad.
He also has GP3 experience under his belt, competing in the 2016 season.
"I'm excited to drive in the F2 championship, especially in my home Grand Prix," Isaakyan said. "This is quite an unexpected development in my season but I'm very thankful for this opportunity and happy to join the team.
“It won't be an easy job as I'll be jumping straight in the car for Sochi practice. The biggest thing will be understanding the tyres – that will be the main focus for me as it was when I did GP3. But I don't think it will be an impossible task to try and get into the top-ten and score some points."
Antonin Charouz, Charouz Racing System owner, added: "We're pleased to welcome Matevos to the team for his home F2 race in Russia and also the final round in Abu Dhabi.
"He has a proven pedigree in single-seater racing and we're excited to see him join the F2 grid. It's always a difficult situation when one driver stands in for another but we're ready to do our best and to finish the season positively."
Bahrain International Circuit - Winter testing