Chelsea have conceded four goals in each of their last two Premier League home games, losing 4-1 to Brentford and 4-2 to Arsenal on Wednesday.
The Blues also lost 3-1 at home to Real Madrid in the UEFA Champions League.
In the middle of those home defeats, Chelsea have recorded impressive away victories at Southampton, Real Madrid and against Crystal Palace in the FA Cup semi-finals at Wembley.
"We have a bit of a bad mix of big individual mistakes and a bit of lack of quality in the one-on-one defensive attitude and behaviour in the box and around the box," Tuchel said.
"We get punished for it heavily. I'm not so sure I have an explanation why. The expected goals we conceded since the national break is a bit higher than normal.
"It's about tactics, high pressing, deep pressing, how chances are created, to keep the expected goals as low as possible."
"Most of the time in football it's a mixture of circumstances and bad luck, and the opponent making the very most of what we offer them," Tuchel said.
"We lack the real determination and attitude on the one-on-one in the box to maybe keep the actual goals under the expected goals."
Chelsea will look to get back to winning ways and plug leaky backline when they meet West Ham United at home in the Premier League on Sunday.
