The Blues lost for the second time in their opening five games despite taking the lead through Raheem Sterling, with Romeo Lavia equalising for the hosts and Adam Armstrong scoring what proved to be the winner on the stroke of halftime.
Chelsea have now lost back-to-back away league games for the first time under Tuchel and have picked up just seven points so far from their five outings.
Tuchel was incensed by the manner Chelsea conceded their goals and has asked for immediate improvement.
"We started well in all the games almost but obviously we are struggling with our focus and consistency in matches," Tuchel told BT Sport.
"We struggle to find answers and find the way back if things don't go in our direction.
"I don't know if concern is the right word. I absolutely dislike to lose, it is the second time this season and I don't think it takes a lot to beat us and this I don't like.
"We try to win matches and the way we do this is something we need to understand as fast as possible. I also don't understand why we are in this situation with injuries to all midfield."
Tuchel continued: "Soft, soft, soft defending. There is no need to give shots away. Just toughen up as a team and show a different mentality.
"I don't like to talk about it normally because you cannot prove it with the data, body position or with the tactical position in defeat. But it's like this.
"Both goals are cheap goals, soft goals, and should not happen if you expect to win a Premier League match, a night game and an away game.
"With our demands and the ambitions we have, it's even more important if key players are missing that we need to show a different mentality - it's too easy to push us off the track, it's too easy to win challenges, it's too easy to bully us."
Chelsea host West Ham United in their next game this weekend.
