Raheem Sterling opened the scoring at St Mary’s, but the visitors had no response to goals from Romeo Lavia and Adam Armstrong.
Tuchel is concerned by his players' mentality in a defeat which would have seen a more emphatic scoreline if not for goalline clearances from Marc Cucurella and Thiago Silva.
“We are not tough enough to win these away matches at the moment,” Tuchel said at Southampton. “It was the same story at Leeds. At Leeds we thought the problem was we didn’t score in the first 25 minutes, and they pushed us off balance with two goals and we couldn’t find any answers.
“Today we scored, everything was okay, but one set-piece again, one sloppy defending again, and we were off balance. We didn’t react. We were lucky not to concede a third goal at the beginning of the second half, and we were not tough enough as a team to react.
“It’s soft, soft, soft defending. What stops l that? Pure mentality. You stop it with pure mentality, defending mentality so there is no superiority for the opponent, just toughen up as a team.
“I don’t like to talk about it normally because you can’t prove it with data or body position, but it’s like this. Both goals are soft goals and shouldn’t happen if you want to win a Premier League match. You need to be tougher than that.”
Chelsea's fitness concerns grew ahead of the game. Reece James was ruled out through illness and joined Trevoh Chalobah, who suffered a knock to his ankle, on the sidelines.
James' replacement on the night, Ruben Loftus-Cheek, limped off at half-time, and adds to a longer-term injury for N’Golo Kante and a suspension for Conor Gallagher.
“It reminds me of Juventus last season,” Tuchel said of the recent spate of injuries. “We played a fantastic match at Stamford Bridge, and after that we had Chilly off for eight months and Kante for eight weeks.
“It was the same against Tottenham this season. We had a fantastic performance and after that our key player says goodbye for six weeks minimum, and then it’s red cards, and Ruben is off injured, and Reece is off with illness.
“It’s key player after key player. But more importantly, if these key players are missing we need to show a different mentality. It’s not enough at the moment to win at Leeds and Southampton.
“We’re humble enough to know we can lose football matches, even if we hate it, but it’s too easy to push us off the track.”
“The line is so thin between excuses and explanations,” he added. “I don’t want to go down this road and give any room for excuses to anybody.
“The closing of the transfer window will help. We will know who has full commitment and what role everybody needs to play and to fulfil, and everybody including myself needs to step up our level of performance. It’s an okay performance, not a disaster, but okay is never good enough.”