The Black Cats climbed off the bottom of the table and into 18th place as a result of their 2-1 win at the Stadium of Light, just one point from safety and only one more adrift of the Foxes, who have now managed a single draw from seven games on the road.
Ranieri's men sat in 15th place on Saturday evening with their domestic form in stark contrast to their return in the Champions League.
Asked what is missing, the Italian said: "It's difficult
We miss everything
Everything last season was right, starting with luck; this season, everything is wrong.
"But we must continue to work hard and maintain the right balance in ourselves, believe in ourselves
The battle is long, but that's football, sport.
"There are good moments, bad moments and in the bad moments, we must stay together as we are.
"Look, I think we play at the same level in the Champions League and the Premier League, but in the Premier League, the first opportunity for the opponent, they score a goal and we have missed chances
In the Champions League, it is different
Maybe the opponents misses a chance and we score a goal.
"It's difficult to explain, but it's the reality because our strength, our effort are the same."
Much of Leicester's success last season was based on Jamie Vardy's goals, and the striker has now gone 16 games for the club, 10 of them in the league, without finding the back of the net, although his manager is not unduly concerned.
Ranieri said: "Look, for me, he works well, hard
Of course the goals are important, but I believe in him and sooner or later, he will start to score goals."
Opposite number David Moyes was thrilled by both his side's performance and the result - a third league win in four attempts - with Robert Huth's own goal and Jermain Defoe's eighth of the season eclipsing Shinji Okazaki's strike.
Moyes said: "The performance was very good, especially in the first 20, 30 minutes
It was probably the best we had played
We just didn't really make our good opportunities into real goalscoring chances.
"To be fair to Leicester, they blocked shots and blocked a couple of things, but I thought we played well.
"I was just a bit anxious
The quality of Vardy and Mahrez on the break, we could never feel totally comfortable while they have that level of player up front.
"If we can win another three out of four, we'll be in a really good position and we'll get ourselves moving the right way.
"We all know we have got a long way to go, but you can only win the games that are put in front of you and at the moment, we are winning them, we are playing better, the team is getting better."
Sunderland were once again indebted to keeper Jordan Pickford, who made a fine late save from Foxes skipper Wes Morgan to preserve their precious lead.
Moyes said: "He didn't have a great deal to do in the rest of the game and I thought today, he was a little bit more jittery than I had seen him.
"But when it came to the moment, he made a top save."
However, Moyes' joy was tempered by the loss of Duncan Watmore with a potentially serious knee injury with fellow youngsters Paddy McNair and Lynden Gooch already facing lengthy spells on the sidelines.
The manager said: "We don't know what it is yet, but the early verdict doesn't sound too good."
Source : PA