Arsenal enters the return leg with a 1-0 deficit, so victory in the series would be a tremendous achievement for the young Gunners, who were given no chance of making it to the final four when the group stage concluded in December.
Defeat would be a crushing blow for Arsene Wenger, who has never won a European trophy in 14 years as Arsenal manager, and revive talk of the possible breakup of the Gunners' young team.
Wenger, who has won 11 English trophies, says what he has or has not accomplished is irrelevant.
"Because for me what is important is what's happening tomorrow," Wenger said. "What is done is done."
He is hopeful of beating Manchester United.
"I will feel very proud of this team if they achieve it because they have been through difficult periods and they have shown great mental strength," Wenger said. "If I had said in November that we will reach the semifinals of the Champions League and FA Cup and go 21 games unbeaten in the Premier League, you would have called an ambulance."
He is counting on his young stars -- including Dutchman Robin van Persie, who is back after missing the first leg with a groin strain -- to get the job done.
"They will," Wenger said. "I'm convinced of that. It's an opportunity for them to come out and be strong, and they will.
Keeping the young Gunners together won't be easy.
The contracts of van Persie and Theo Walcott both expire at the end of next season. There is the possibility that Real Madrid or Barcelona will make a move to acquire young Spanish star Cesc Fabregas, whose contract with Arsenal extends to 2014.
Wenger went so far as to call his commitment to keeping his young players together "an obsession."
"When young players get a chance like this somewhere," he said, "there has to be a loyalty to stay together. That is why we created that policy."
Fabregas, for one, says he'll stay.
"My future belongs to Arsenal," he said.
1-Manuel Almunia (Spain, 31)
3-Bacary Sagna (France, 26)
5-Kolo Toure (Ivory Coast, 28)
18-Mikael Silvestre (France, 31)
40-Kieran Gibbs (England, 19)
2-Abou Diaby (France, 22)
17-Alex Song (Cameroon, 21)
14-Theo Walcott (England, 20)
4-Cesc Fabregas (Spain, 21)
8-Samir Nasri (France, 21)
25-Emmanuel Adebayor (Togo, 25)
1-Edwin van der Sar (Netherlands, 38)
22-John O'Shea (Ireland, 28)
5-Rio Ferdinand (England, 30)
15-Nemanja Vidic (Serbia, 27)
3-Patrice Evra (France, 27)
24-Darren Fletcher (Scotland, 25)
16-Michael Carrick (England, 27)
8-Anderson (Brazil, 21)
7-Cristiano Ronaldo (Portugal, 24)
32-Carlos Tevez (Argentina, 25)
10-Wayne Rooney (England, 23)
Referee: Roberto Rosetti (Italy)