Injuries to three key players could derail Arsenal's attempt to qualify for the knockout stages of the Champions League when it plays Olympiakos on Wednesday.

Arsene Wenger's team must win, either by a two-goal margin or by one goal if it scores at least three times, to qualify for the last 16.

But the absence of striker Alexis Sanchez to a hamstring problem increases the difficulty for Arsenal, while the core of its midfield will also be ruled out.

Creative midfielder Santi Cazorla has undergone knee surgery, while Francis Coquelin is unable to play in his defensive midfield role due to a knee ligament problem.

Despite the loss of three crucial players, Wenger will instruct his side to go on the attack.

''We know exactly what we have to do,'' Wenger said. ''We know that we have to go for it from the start. We know what is expected and we have studied Olympiakos.''

Elsewhere in Group F, Dinamo Zagreb hosts Bayern Munich, which has already sealed its place in the last 16 of the competition.

Here are some things to know ahead of Wednesday's games:

Campbel returns to Olympiakos

Joel Campbell has not been a regular choice for Wenger at Arsenal, but the injury to Sanchez means the Costa Rica attacker could start against Olympiakos, the team he played for on loan in the 2013-14 season.

Campbell opened the scoring in Arsenal's 3-1 victory over Sunderland on Saturday and is relishing the chance to try and keep Arsenal in Europe's most prestigious club competition.

''It is the most important game of the season so far,'' Campbell said. ''We have to prepare mentally and physically for Wednesday's game because it will be very difficult.''

Theo Walcott made a substitute appearance against Sunderland, his first appearance since injuring his calf in October and could feature in Greece.

Ribery back for Bayern

Having been sidelined with an ankle injury since March, Franck Ribery is finally back in action and could play against Dinamo.

The 32-year-old winger joined team training on Wednesday and on Friday traveled with the squad to play Borussia Moenchengladbach. He came on for the last 15 minutes and scored Bayern's only goal, six minutes after leaving the bench, but his team were surprisingly beaten 3-1.

''It feels good to be sent if you've been sidelined for so long,'' Ribery said. ''"The coach asked me if I wanted to travel to 'Gladbach because I'd trained so well. I said, of course, count me in. But I still need time, rhythm, good training.''

Coach Pep Guardiola was equally pleased.

''Nine months is a long time. But he played with his usual aggression straight away,'' Guardiola said.

Bayern has already qualified for the last 16 of the tournament but Guardiola will be eager for his team to return to winning ways against Dinamo.

Olympiakos need concentration 

Olympiakos is cruising toward another domestic title, but its latest win rattled the Greek champion.

Coach Marco Silva's men earned their 12th straight win, 4-3 over Panathinaikos, having only conceded five times in the previous 11 league matches.

''Obviously I don't like conceding goals - certainly not three goals,'' Silva said. ''It shows a lack of concentration and it's something we are addressing.''

The Greeks will be chasing an early strike at Karaiskakis Stadium on Wednesday, hoping to wrong foot Arsenal.

Top league scorer Kostas Fortounis netted his 10th goal of the season on Saturday. Silva's only injury worry is Colombian Felipe Pardo, who gave the Greeks a huge boost last month, scoring twice in the 2-1 win over Dinamo Zagreb.

Dinamo wants to end on a high

Dinamo began its Champions League campaign with a surprising but impressive 2-1 victory at home to Arsenal. Since then, it has lost every game in the tournament.

Regardless of the result at home against Bayern, it will finish last in Group F and a victory over Guardiola's side would be its first against a German opponent.

When Dinamo played Bayern in Germany in the second group match, Bayern surged to a 5-0 victory.

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