Fresh off a first English league title in 44 years, Manchester City was supposed to make its presence felt among Europe's elite this season with its expensively assembled squad.
A slip-up in the UEFA Champions League this week, however, will leave the English club facing the prospect of another campaign in the Europa League - or worse, dropping out of Europe altogether.
Of the continent's top sides, City is under the most pressure heading into the third round of group-stage matches in the Champions League.
With just one point from two matches so far, failing to beat Ajax on Wednesday (Thursday AEDT) could leave City with too much to do to qualify from a tough Group D that also contains Real Madrid and Borussia Dortmund.
City also failed to progress from the group stage last season, although that was put down to a lack of experience at this level.
Much more is expected this time.
"I don't know if it is a learning curve - we are in with the big boys now, there is no place to hide," City goalkeeper Joe Hart said. "There is no point learning if you are not going to win. We need one big game against Ajax next."
There are no such concerns for competition heavyweights Madrid, Barcelona, Manchester United and Arsenal, who are all looking for their third straight group wins.
As are BATE Borisov, the Belarusian team that inflicted a shock 3-1 loss on last season's runner-up, Bayern Munich, last time out and next hosts Valencia in Group F.
Bayern visit bottom-place Lille and will look to transfer its domestic form to the Champions League after becoming the first side to start a Bundesliga campaign with eight wins.
"We have to keep it up and ... be ready for Lille," Bayern winger Franck Ribery said.
Malaga, who is making its debut in the competition, and FC Porto are the other two teams to have made 100-per-cent starts in the competition.
The group involving City and Real Madrid has been given top billing this season, but while the English champion has struggled to get going, its Spanish counterpart has been in fine form and has scored seven goals.
However, Jose Mourinho's team heads into its tough away match at Dortmund with defensive problems because of injuries to fullbacks Marcelo, Alvaro Arbeloa and Fabio Coentrao. Michael Essien, a central midfielder, was forced to fill in at left back in the 2-0 win over Celta Vigo at the weekend.
"We are missing players, but all of us in the squad can play in various positions and that is what we are doing," said Madrid forward Jose Callejon. "We have had to relocate players, but it has been well done."
Borussia Dortmund will be without Jakub Blaszczykowski (left knee) and has doubts over the fitness of Germany midfielders Mario Goetze (thigh) and Ilkay Guendogan (spine) as it looks to rebound from the weekend's 2-1 derby loss to Schalke, which left the champion already 12 points behind Bayern.
Barcelona is at home to Celtic and will look for more inspiration from Lionel Messi, whose hat trick in the 5-4 win over Deportivo La Coruna on the weekend took his tally to 15 goals for the season and 71 for club and country in 2012. He is only four short of equalling Brazil great Pele's record in a calendar year from 1959.
"Statistics are just numbers," said Messi. "I am only concerned with winning games."
Celtic's surprise 3-2 win at Spartak Moscow last time out put the Scottish champion second in Group G, and it has won its last six games in all competitions.
In Group E, defending champion Chelsea visits Shakhtar Donetsk in a match between the joint leader on four points, while Juventus - two points behind after two straight draws - is away to FC Nordsjaelland.
Manchester United is at home to Braga as Alex Ferguson's team look to stay in control of Group H, and Arsenal hosts Schalke - the top two in Group B and likely qualifiers for the knockout stage.
English Premier League champion Manchester United will start the 2013-2014 season at League Cup winner Swansea City before facing Chelsea and Liverpool.