LONDON: Arsene Wenger has told his Arsenal stars to forget about a looming showdown with Chelsea and the absence of injured striker Robin van Persie as they look to book their place in the knockout stage of the Champions League. A point at home to Standard Liege on Tuesday will ensure the Gunners of progression to the last 16 while victory over the Belgian champions would guarantee them top spot in group H. That outcome would provide a welcome tonic five days ahead of Chelsea's visit to the Emirates for an encounter that looks like being pivotal to Arsenal's prospects of sustaining their Premier League challenge. An unexpected defeat at Sunderland on Saturday has inevitably been interpreted as a sign of trouble along the road for Arsenal following the loss of top goalscorer van Persie to an ankle injury that will keep the Dutchman sidelined for up to six weeks. It has also significantly increased the stakes involved in the meeting with Chelsea, who will kick off with an eight-point advantage over their London rivals. "We have to forget about Chelsea," Wenger said. "It's important to win our next game and that is in the Champions League. "Against Sunderland, I don't think we missed anybody especially. We lost a game where we can only fault ourselves because we didn?t score, some players were not at their best and we gave a goal away on the only situation where Sunderland could score, a set piece." The defeat at Sunderland was Arsenal's first in 14 matches but centreback Thomas Vermaelen does not believe it will seriously reduce the momentum the squad have established this season. "It is important to stay focused, there are a lot of important games coming up," the Belgian defender said. Czech midfielder Tomas Rosicky drew a line under 18 months of injury agony when he made his first start for Arsenal since January 2008 in the opening match in the group, a 3-2 win in Liege. And the 29-year-old has warned the Belgians that he will pose a much greater threat to them on Tuesday than he did on that night, when Arsenal had to fight back to take all the points after leaking two early goals. "Fitness wise I have definitely improved in recent weeks," Rosicky said. "I do not feel exhausted after the games or anything like that. "In the matches I have started, I have not felt tired out even if the boss has decided to take me off, so I clearly have a good base of fitness to work with. "I know there are some aspects I need to improve on, and they will get better as I play more and more so I am not worried about that. "For example, a bit of explosiveness is something that I am still missing a little, but I know it will improve as I play more and will arrive naturally."