Macheda, 17, came on as a substitute in the last two Premier League matches to score crucial winners against Aston Villa and Sunderland.
But United will have to tighten up in defence, be more creative in midfield and overcome mental tiredness to eliminate Porto, who surprised many people with their pace and fighting spirit in last week's 2-2 draw in Manchester.
United recovered from going behind to an early Cristian Rodriguez goal and led 2-1 with five minutes left following goals by Wayne Rooney and Carlos Tevez.
But a lack of defensive concentration allowed Mariano to equalise in the dying minutes.
"We only have ourselves to blame. It just exemplified the fragile nature of our defending at the moment," United manager Alex Ferguson said.
Ferguson's main concern is the sudden fragility of his defence which looked all but impregnable this season until Liverpool thrashed them 4-1 at Old Trafford a month ago.
United have since conceded seven goals in four matches and Ferguson will hope England defender Rio Ferdinand recovers from a groin injury.
Ferguson should be able to field striker Dimitar Berbatov who has recovered from an ankle which ruled him out of the first leg.
"It's a difficult job now," Ferguson said of his team's bid to become the first English team to win at Porto's Dragao Stadium.
PORTO CONFIDENT
Porto coach Jesualdo Ferreira said the quality of his side's display at Old Trafford was even more important than the two away goals.
"I highlight the performance which gives us confidence and optimism. There were periods in which we were brilliant," he said.
Plaudits for the team's fearless show have left the club brimming with confidence that they can eliminate United again, just as they did on route to winning the trophy under Jose Mourinho in 2004 but no one is getting too carried away.
"The 2-2 result is very positive but there are still 90 minutes left. If we want to reach the semi-finals, we have to be humble as Manchester United have a lot of quality," captain Lucho Gonzalez said.
Possible teams:
Porto: 1-Helton; 2-Bruno Alves, 14-Rolando, 21-Sapunaru, 28-Cissokho; 8-Lucho Gonzalez, 16-Raul Meireles, 25-Fernando; 9-Lisandro Lopez, 10-Cristian Rodriguez, 12-Hulk
Manchester United: 1-Edwin van der Sar; 22-John O'Shea, 5-Rio Ferdinand or 23-Jonny Evans, 15-Nemanja Vidic, 3-Patrice Evra; 24-Darren Fletcher, 16-Michael Carrick, 18-Paul Scholes; 7-Cristiano Ronaldo, 10-Wayne Rooney, 9-Dimitar Berbatov
Referee: Massimo Busacca (Switzerland)
