Manchester United and Arsenal are in Champions League action this evening, and Chief Sports Writer Scott Wilson expects supporters of both to be biting their fingernails as they attempt to make the last four.

ARSENAL (1) v VILLARREAL (1) (Sky Sports 3)

IT has been a difficult season at the Emirates Stadium, but football being football, it could yet turn out to be a great one.

By Monday morning, Arsenal could be in the FA Cup final and the semifinals of the Champions League.

Emmanuel Adebayor’s acrobatic strike in Spain last Tuesday made the latter much more likely, and the Gunners have also been boosted by events in the wake of last week’s first leg.

Influential midfielder Marcos Senna, the scorer of Villarreal’s goal in El Madrigal, injured his hamstring in Saturday’s defeat to Malaga and is out of tonight’s second leg.

He joins an injury list that already includes the talented Santi Cazorla, and with Arsenal unbeaten in 23 home matches in the Champions League, it is hard to see Villarreal overcoming the absence of their most important player.

Arsenal have problems of their own, particularly in defence where William Gallas has been ruled out for the season, but Arsene Wenger’s side should be strong enough to reach their first Champions League semi-final since 2006.

PORTO (2) v MAN. UNITED (2) (Sky Sports 2)

PORTO have never lost a European home match against an English club. So unless they are going to be involved in a thrilling 3-3 draw, Manchester United will have to create history if they are to keep their hopes of defending the trophy they won last May alive.

Sir Alex Ferguson’s side have been rocking this month, with last week’s draw with Porto exposing worrying frailties that do not augur well.

They will have to improve considerably if they are to make the last four, but the return of Rio Ferdinand could prove crucial given last Tuesday’s defensive howlers.

United will attempt to prevent Porto’s quick passing from carving them open at the Estadio de Dragao.

Porto, who are unbeaten in 20 matches, knocked Manchester United out of the Champions League in 2004, when a certain Jose Mourinho galloped down the touchline at Old Trafford.

They are favourites to do so again this evening, but with Cristiano Ronaldo, Wayne Rooney and Dimitar Berbatov all expected to start, United will kick off with confidence.