Chelsea take on Porto on Wednesday night needing to avoid defeat in order to qualify for the knockout stages of the Champions League - something that Blues fans would have felt was assured when the groups were announced in September.
And in the build-up to the crunch match, Porto keeper Iker Casillas has insisted the Portuguese side will do their best to make Chelsea 'suffer' according to the Sunday Mirror.
Both sides are currently top of Group H with ten points each, however, the Londoners are ahead by virtue of goal difference. Should Chelsea - knowing they need to avoid defeat - suffer a loss to Porto, the Blues will be confounded to Thursday night football and playing in the Europa League instead.
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Things are a little bit more complex for Porto, who will either need to beat Chelsea, or hope Dynamo Kiev fail to win against Maccabi Tel Aviv to avoid dropping into the Europa League.
Casillas - who had a torrid time whilst under the management of Mourinho at Real Madrid - is confident his side will go to Stamford Bridge and provide Chelsea with a real test.
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"Porto has not prepared for a defensive match because that will be a disaster and almost certainly result in a defeat," Casillas told the Sunday Mirror.
"We're obsessed with staying in the Champions League.
He added: "Mourinho is a coach who will always want a victory and now against his old club. But we have the mentality that we can make them suffer at Stamford Bridge and get through the group phase."
"To be out of the Champions League in the next few days would be a big blow. Chelsea are not playing with the same level that they played in the past seasons, but the Blues have an enormous squad that is capable of beating any European rival."
Casillas who, up until this season, had spent his entire career at Madrid, has plenty of experience playing in England and knows the atmosphere can be a difficult factor for a foreign team.
"It's never easy to play English clubs in their own stadiums with the atmosphere there. But this match is decisive for us."
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