City will have to overcome a 1-0 deficit to reach the quarter-finals after losing to a Xandao goal in the first leg against the Portuguese side in the Jose Alvalade Stadium last night [Thursday].
With City top of the Barclays Premier League and Sporting fourth in their domestic competition, the result was unexpected but De Jong believes the English club are still the favourites.
The Holland international, who returned to the side in place of suspended talisman Yaya Toure, said: "It wasn't the best performance - we were lacking a little bit of energy.
"The tempo was too slow and it doesn't matter what opponent you have in front of you, you can always give them chances to get back in the game, and you could see that last night [Thursday].
"We are lucky we have two games, and everyone knows how we play at home, and we are confident we can go through to the next round.
"All credit to Sporting as they played their hearts out, but quality-wise we will just have to see when we are at home."
City boast a formidable home record having won all 14 of their Premier League games at the Etihad Stadium this season.
Yet they were below par in Portugal and undone when Brazilian defender Xandao reacted quickly to backheel past Joe Hart after a double save from the goalkeeper early in the second half.
Hart prevented further damage with fine saves from Joao Pereira and Ricky van Wolfswinkel among a host of other Sporting chances.
City went close through Gareth Barry and Aleksandar Kolarov but it was not until the late introduction of Mario Balotelli they began to look lively, the Italian setting up a narrow David Silva miss.
De Jong, 27, said: "They dropped back quite a lot but that's how a lot of opponents see us nowadays, as a big club, a big target and a big threat, so they come with 11 men behind the ball and from there try to counter us.
"We had most of the possession but that doesn't win you games - it's scoring goals that wins you games."
City are now staying at their Portuguese training base until travelling directly to south Wales tomorrow [Saturday] for Sunday's Premier League game against Swansea.
Roberto Mancini's men will hope to quickly recover their vitality as they look to protect a two-point lead at the top of the table.
But they will have to make do without influential captain Vincent Kompany, who suffered a calf injury last night which the club fear could keep him out for at least two weeks.
Fellow defender Pablo Zabaleta is already out but Joleon Lescott is expected to overcome a groin soreness while right-back Micah Richards should also be fit to return.
De Jong said: "We lost Vinnie and were already without Zaba and Joleon was struggling a little bit, so it's a big blow."
Mancini, who will be boosted by the return of Toure at the Liberty Stadium, does not think his players will have any difficulty switching focus back to the title race.
He said: "The Europa League and Premier League are different competitions.
"Now we have another 90 minutes for the Europa League next week, in the Premier League we have another 11 games.
"Every game will be important for the title race but Swansea is the next game, so it is the most important."
Earlier this season Mancini described Swansea as the best footballing side in the Premier League and has remained impressed by the promoted Welshmen.
He said: "They are playing very well, they play good football.
"They started the season very well and they have continued to play good football. I think it will be a tough game."
Swans boss Brendan Rodgers was recently mentioned as a potential Chelsea manager and Mancini believes he has the ability to go far in the game.
Rodgers' side have acquitted themselves well in the their first season back in the top flight and are 11 points clear of the relegation battle.
Mancini said: "I think that he is a good manager.
"It is clear it is totally different, playing for the title compared to avoiding relegation, but I think he has the capacity."