Surely Tuchel meant that having want-away players and lots of uncertainty over transfer dealings, both incoming and outgoing, can dampen the mood of the whole team and have a negative effect on the overall atmosphere at the training camp. If certain players, especially when one of them is the club captain, start giving only 90% effort and just going through the motions it can very quickly turn into a slippery slope and affect some of the other players too. It probably shouldn't affect it that much but it can still happen and it's at least partially Tuchel's own fault as well because he's the leader and should probably have realized sooner if there's a problem and try to do more to correct it.
No matter how bad and frustrating the performance was I'm quite sure he wasn't talking about last night's game in isolation but rather about the bigger picture.
Trust me, I've played enough FM in my life to know how one player bitching about something can all of a sudden turn into a huge squad morale problem.