Europe's big guns have emerged largely unscathed from the early skirmishes in this season's Champions League group stages and several can virtually guarantee their places in the knockout round this week.

Group C leaders Barcelona should make it four wins from four matches against pointless Basel on Tuesday, holders Manchester United [Images] can book their last 16 place with victory at Celtic while Chelsea know a win at Roma should be enough.

Inter Milan, Bayern Munich and Arsenal [Images] are also in strong positions going into the fourth round of fixtures, but the likes of Porto and PSV Eindoven have plenty still to do.

Porto, champions of Europe in 2004, will be eliminated if they lose in Kiev on Wednesday, while Dutch representatives PSV need a win in Marseille to keep on the coat tails of Group D leaders Liverpool and Atletico Madrid who meet at Anfield.

Nine-times European champions Real Madrid [Images] still have their destiny in their own hands in Group H, although they trail Juventus by a point after defeat in Italy [Images] two weeks ago.

The Spaniards will be eager for revenge and top spot in Wednesday's return in the Bernabeu.

Roma look the most vulnerable of the fancied sides after an erratic opening to their Group A campaign that began with a shock home defeat by Romanian debutants CFR Cluj.

That defeat, and a narrow loss to Chelsea two weeks ago, has left them with just three points so far and another reverse at home to Chelsea tomorrow could spell the end of their hopes of reaching the final in the Eternal City.

Their form of Luciano Spalletti's team hardly bodes well. A 2-0 defeat by Juventus on Saturday, their fourth consecutive loss in Serie A, left them perched just above the relegation zone and with their confidence in tatters.

Chelsea, meanwhile, have recovered from losing to Liverpool last week to hit eight goals in two games without reply to reclaim top spot in the Premier League.

Ashley Cole is unlikely to make the trip to Rome after injuring his calf in the 5-0 victory over Sunderland on Saturday although manager Luiz Felipe Scolari [Images] has an enviable decision to make regarding his strikers.

Didier Drogba [Images] is fit again after a knee injury but Scolari said Nicolas Anelka [Images] will start after scoring a hat-trick against Sunderland.

''My big decision against Roma is to start Anelka,'' Scolari told Chelsea's website (www.chelseafc.com). ''He is in good condition now so I will play him.'' Defeat for Roma would also open the door for Cluj to get one foot in the door of the last 16 if they beat bottom club Bordeaux.

Another unfancied club has the chance to continue ruffling feathers when Cypriots Anorthosis Famagusta host Italian champions Inter Milan still very much in the hunt for a place in the knockout phase.

Famagusta are second to Inter with four points but Werder Bremen are just a point behind and the Germans will be confident of overhauling the islanders if they beat Panathinaikos at home.

The top two from each of the eight groups go through to the second round with the third-placed team entering the UEFA [Images] Cup.