(ESPN2, ESPN360.com, 2:45 pm ET)
Manchester United -- 1-Edwin van der Sar; 22-John O'Shea, 23-Jonny Evans, 15-Nemanja Vidic, 3-Patrice Evra; 7-Cristiano Ronaldo, 16-Michael Carrick, 18-Paul Scholes, 11-Ryan Giggs; 32-Carlos Tevez, 10-Wayne Rooney.
Porto -- 1-Helton; 2-Bruno Alves, 14-Rolando, 21-Sapunaru, 28-Cissokho; 8-Lucho, 16-Raul Meireles, 25-Fernando; 9-Lisandro Lopez, 10-Cristian Rodriguez, 12-Hulk.
FORM. Manchester United returns to action only 48 hours after it 3-2 victory over Aston Villa kept it in first place in the English Premier League. It's won two titles this season -- Club World Cup and English League Cup -- and is in contention for three more championships.
United returns Wayne Rooney, Nemanja Vidic and Paul Scholes from suspension. Injuries will keep Rio Ferdinand, Anderson and Dimitar Berbatov out the lineup.
Porto extended its unbeaten Portuguese league run to 16 games with a 3-1 win at Vitoria Guimaraes.
MATCHUP. Vidic will have to keep his eye on Brazilian Hulk, one of the most impressive players in this season's Champions League.
HISTORY. United and Porto have split two Champions League series. United knocked Porto out of the 1997 Champions League in the quarterfinals; Porto defeated United in the second round en route to its 2004 championship.
(Setanta Sports USA, ESPN Deportes, ESPN360.com, 2:45 pm ET)
Villarreal -- 13-Diego Lopez; 18-Angel Lopez, 4-Diego Godin, 2-Gonzalo Rodriguez, 5-Joan Capdevila; 10-Cani, 19-Marcos Senna, 6-Sebastian Eguren, 7-Robert Pires; 15-Nihat Kahveci, 22-Giuseppe Rossi.
Arsenal -- 1-Manuel Almunia; 3-Bacary Sagna, 5-Kolo Toure, 10-William Gallas, 22-Gael Clichy; 27-Emmanuel Eboue, 17-Alex Song, 15-Denilson, 4-Cesc Fabregas; 14-Theo Walcott, 25-Emmanuel Adebayor.
FORM. Arsenal has played its best soccer of the season, running off five straight wins as part of a 17-game unbeaten streak. Arsenal will be without strikers Robin van Persie and Eduardo and midfielder Abou Diaby, who all are out with groin injuries. Samir Nasri (virus) is a question mark.
Villarreal is coming off a 3-0 loss to Almeria. The defeat was costly as winger Santi Cazorla broke a bone in his right foot and will miss the rest of the season.
MATCHUP. It will be a battle of Spanish national team stars in midfield. Villarreal should have Brazilian-born Marcos Senna back after sitting out Saturday's game against Almeria with an ankle injury. Arsenal captain Cesc Fabregas returned from a long layoff due to a knee injury Saturday and assisted on both of Emmanuel Adebayor's goals in the Gunners' 2-0 victory over Manchester City.
HISTORY. Arsenal reached the 2006 Champions League final with a 1-0 aggregate win over Villarreal in the semifinals. The series wasn't decided until Arsenal goalkeeper Jens Lehman saved Juan Roman Riquelme's late penalty in the 0-0 tie in Villarreal.