Seven-times European champions AC Milan are looking to bounce back from a disastrous start to their Serie A campaign as they go in search of a place in the Champions League group stage this week.
The Italian giants and 2006 Champions League finalists Arsenal are amongst 20 teams in the play-off round vying for a spot in the lucrative group stages.
Milan, who last won the competition in 2007, go into their return fixture against Dutch runners-up PSV Eindhoven at the San Siro with a slight advantage after a 1-1 draw in the Netherlands a week ago.
However, Milan's domestic campaign got off to a dreadful start at the weekend as they crashed to a 2-1 defeat at newly-promoted Verona, with coach Massimiliano Allegri lashing out at his side's display.
"We had the totally wrong attitude (on Saturday) and we're not going to make much progress if it continues," warned Allegri.
Midfielder Andrea Poli emphasised the need to put the setback behind them and focus solely on Wednesday's match against PSV, a game Allegri labelled as the "most important of the season so far".
"We had a positive performance at PSV and we faced a dangerous side, but we got a good result and we will look to finish the job in the return leg," said Poli.
"We always have respect for the opposition, but we are Milan and we have to qualify at every cost. We have to stop talking about it and just get on with the job in hand," added the Italy international.
Arsenal effectively ensured the Premier League will have four representatives in this season's competition after a commanding 3-0 victory against Fenerbahce in Turkey put Arsene Wenger's men firmly in the driving seat ahead of Tuesday's second leg in London.
History overwhelmingly favours Arsenal, who are on the brink of a 16th straight appearance in the group stage, with only two teams ever having inflicted a home defeat of three or more goals on the Gunners in the club's European history.
Further lengthening the odds of an unlikely comeback is Fenerbahce's dismal record in England, where they have scored a combined three goals in eight visits.
However, left-back Kieran Gibbs was adamant Arsenal would not become complacent, despite their status as heavy favourites to advance.
"It's still not over because we've got another tie," said the defender.
"We have to go in on all cylinders again, otherwise if we sit back and think that we're already through it can put us on the back foot, and we don't want that. We have to be strong and make sure we finish off the job."
1967 winners Celtic must overturn a two-goal deficit against Kazakh champions Shakhter Karagandy in Glasgow on Wednesday if they are to continue their Champions League adventure.
"The tie is far from over. We've got plenty in us to turn the deficit around and the players will be very, very motivated," said Hoops boss Neil Lennon.
"There will be a few players who will be disappointed with their performances, but we're still well in this tie, although we've got a lot to do," he added.
The Scottish club can take solace from the fact that they have staged a successful comeback from a 2-0 first-leg deficit in Europe on three of four occasions at Celtic Park.
German side Schalke face a testing trip to Greece to face PAOK Salonika after a 1-1 draw in Gelsenkirchen, while Polish champions Legia Warsaw host Steaua Bucharest after a 1-1 draw in the Romanian capital.
Real Sociedad, Austria Vienna, Basel and Viktoria Plzen go into their second-leg fixtures with two-goal leads, while Zenit St Petersburg take on Portuguese side Pacos Ferreira, with the Russians up 4-1 from the first leg.