The first leg ended in a 1-0 win to the Blues at Stamford Bridge and Marchionni believes the Old Lady have a “30 per cent chance” of beating Chelsea.
“There is a lot at stake because we have the derby against Torino, which means a lot to the fans,” Marchionni told Sky Sport Italia.
“That is also a vital game in our Scudetto bid. And then we have the most important match of the season – the second leg of our Champions League last 16 game.
“However, we aren’t letting the Chelsea game interfere with our preparations for the derby.
“We must keep focused because we want to fight Inter until the wire and moving two points closer has given us hope.
“What are our chances in the Champions League? We lost the first game so they have an advantage, but I’d say we have a 30 per cent chance and they have 70 per cent.
“We have a good chance of getting through and we have to prove that on Tuesday because we are playing to save our season.”