Arsenal expect to learn on either Thursday or Friday whether the potentially decisive Champions League group tie against Napoli at the Stadio San Paolo will be played behind closed doors.

Uefa have almost concluded their investigation into the crowd trouble that marred Napoli's home win against Marseille two weeks ago and the fear in Naples is that the governing body will make an example of them and impose a ban on all supporters for the final Group F fixture against Arsenal on 11 December.

Arsenal do not expect Uefa to order the exclusion of fans and are pressing ahead with their usual plans to look after the supporters who want to travel.

Napoli's 3-2 win over Marseille put them joint top of the group with Arsenal but there was fighting between rival fans outside the San Paolo. Ten people were injured and riot police had to intervene. Flares were set off inside the stadium and there were complaints about Napoli's organisation of the event.

The San Paolo creates one of the most passionate atmospheres in European football and Napoli would not want to be without their fans for a tie that could hold the key to their qualification. Before Arsenal's visit, the Italian club travel to Dortmund to play Borussia on Tuesday of next week. Arsenal face Marseille at home on the same evening. Dortmund sit three points behind Arsenal and Napoli.

Wojciech Szczesny, meanwhile, is confident that Arsenal's current squad is strong enough to extend their excellent start to the Premier League campaign and challenge for the title without further significant strengthening during the January transfer window.

The London club's defeat at Manchester United just before the international window trimmed their lead at the top to two points ahead of the visit of third-placed Southampton to the Emirates Stadium on Saturday. That loss was only a second in 11 games this term, and a fifth in 30 matches over the calendar year, with the team's new-found consistency cause for future optimism.

Arsenal have clearly benefited this term from the club record £42.5m arrival of Mesut Özil from Real Madrid, with Arsène Wenger expected to add further to his options in January to maintain the club's challenge.

The Schalke midfielder Julian Draxler, a long-standing target, has indicated he would not be willing to move mid-season but the London club are expected to pursue a forward to compete with Olivier Giroud, and interest is retained in Real Madrid's Karim Benzema.

"I wouldn't say we have proved anyone wrong and passed all the tests, and I can't say that until we get our hands on a trophy in May," Szczesny said. "But what is important is what we believe in, and we believe that we are good enough with the players we have got and the work we are doing to win the Premier League.

"We have shown that to people over the last six months or so. We know that if we just carry on doing the same thing, we will show people that this team and these players are good enough to win a trophy without any extra additions or the extra work people are talking about. We certainly believe we can do it.

"It's been almost 12 months really because it is not only this season we've been doing well. It was the last two or three months of last season, where we went unbeaten for a few games [they lost once in 16 league games].

"We are confident that we see the quality is there and we know that, when we play at our very best and do it consistently well, we can beat anyone, basically. A couple of years ago there was a case of not being consistent enough, individually or as a team, but now you can see that whoever comes into the team does a fantastic job. It seems to be bringing results. If we are at our best we will win a trophy."

Southampton, unbeaten in eight league games stretching back to August and a side who have already won at Liverpool and drawn at Old Trafford, will provide a stern test of those credentials at the Emirates with their trio of England players – Rickie Lambert, Adam Lallana and Jay Rodriguez – all expected to feature.

"But we have to make sure we keep our form going and not just rest on our laurels and think we've done it now," added Szczesny. "We need to make sure we continue doing what we have been doing recently and keep getting the results."