The Nerazzurri held the Scottish champions to a 3-3 draw in Glasgow last week in the first leg of their round of 32 clash.
“We need maximum concentration for this game,” Mancini told the Italian media on Wednesday. “The 3-3 result in the first leg gives us a slight advantage.
“But we need to be fully focused because Celtic is a type of team that never gives up.
“Tomorrow’s match is important. It’s not going to be an easy game and we cannot make any mistakes. We’ve had the best possible preparation thanks to our good results.”
Inter endured a dismal start to the season, one that knocked them out of the Scudetto race. They were eliminated from the Coppa Italia last month, yet the team appears to have turned the corner under Mancini.
Goals from Mauro Icardi and Mateo Kovacic handed Inter a 2-1 triumph at Cagliari on Monday, Inter’s third straight league win. The success lifted the Nerazzurri to eighth place in Serie A.
Not since November 2012 had Inter won three straight league games in a row.
“The victories give us confidence,” Mancini said. “Our three wins in a row are down to hard work. “For now, we don’t look at the table, because we are not still where Inter should be. “We need to keep playing well.
“We need to improve the way we manage games, especially when we’re in the lead.”
Inter are unbeaten at home this season in European competition. “The fans at the San Siro have always helped us,” Mancini said. “I believe we will have a good crowd tomorrow that will give us a helping hand.”
Juan Pablo Carrizo will replace Samir Handanovic between the posts with the Argentinian as Mancini’s chosen goalkeeper for the Europa League.
Mancini confirmed that Rodrigo Palacio, who scored two goals in the opening leg last week, will play alongside Icardi in attack, with Fredy Guarin playing in support.