The Portuguese midfielder joined Chelsea from Barcelona in 2008 and featured 57 times for the club across two seasons, the second of which a title-winning campaign under Carlo Ancelotti.
The Stamford Bridge outfit currently sit 15th in the league having lost five games already this campaign, but Deco is confident Chelsea can still muster a title bid.
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Speaking to BBC Sport, Deco said: "In my opinion, the Premier League is stronger than in the past few years.
"It is difficult. All the clubs have a lot of high-quality players. But the season is still long. I think Chelsea can still win the title.
"Of course, it is difficult when you are losing games. But I think a gap of nine or ten points can still be closed in the Premier League."
The 38-year-old also voiced his support for under-fire boss Jose Mourinho, whom he won the Champions League and worked with during his days at Porto.
“Mourinho is one of the best coaches in the world. He is one of the best I have worked with," he continued.
"He knows his team and he knows how to play because they won the title last season. He knows his players very well.”
Chelsea's chances look bleak, of course, following a number of poor results, though it remains difficult to completely rule out their chances given the undoubted talent they have in their ranks.
However, whilst scrutiny and lacklustre performances continue to plague the Premier League champions, it remains to be seen how the Blues plan on breaking out of their ongoing slump.