The pair have eight points in Group C, with United ahead on goal difference, and victory for either at Old Trafford would put that side through as group winners with a game to spare. A draw would send both through if FC Basel lose to Otelul Galati.
Sir Alex Ferguson's United have tightened up since last month's 6-1 drubbing by Premier League leaders Manchester City, with the manager saying that game had been a "wake-up call" that has been followed by five wins in a row with clean sheets.
"A few weeks ago there was genuine criticism about the defending and the chances opponents were getting," Ferguson told MUTV after Saturday's 1-0 league win at Swansea City.
"When you win these games 1-0 it brings the concentration back to a good level which has been lacking. The concentration we showed today is exactly the concentration we need back in the team."
The return from injury of captain Nemanja Vidic to marshal the heart of defence has coincided with the improvement but United will not be able to count on him against Benfica while he serves the second of a two-match ban.
In his absence, Rio Ferdinand is likely to be partnered by Jonny Evans as last season's runners-up seek to qualify from the group stage for a sixth consecutive year.
United have also had problems in midfield due to injuries, with Ferguson resorting to deploying striker Wayne Rooney in a central role in this month's 2-0 win over Galati in their last European game, but they are now looking in better shape.
"Michael Carrick was outstanding," Ferguson said of the midfielder's first league start of the season at Swansea. "I thought he did really well - it was a big performance from him.
"To get Michael back to that form and have the likes of Ryan [Giggs] available gives us good options."
OLD RIVALS
Benfica are unbeaten on their European travels this season but that may count for little at Old Trafford, where United have lost only once in their last 34 Champions League games there dating back to 2005.
The clubs, who are old rivals with United having beaten the Portuguese side when they won their first European Cup in 1968, drew 1-1 in Portugal in their group opener in September.
The recent performances of Spain Under-21 international Rodrigo will give Benfica coach Jorge Jesus a welcome headache over who to pick to spearhead Benfica's attack on Tuesday.
The 20-year-old has stolen the show in their last four matches, scoring each of his team's five goals and standing out with his above-average flair and awareness.
"I'm going through a good moment but only God knows how far I will go. I don't know if the coach will make me a regular first-choice player, that's for him to decide but two years ago I'd never have imagined I'd be here," Rodrigo said.
Paraguay's Oscar Cardozo nevertheless remains the natural first choice up front and is Benfica's top scorer in the league and Champions Leag
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