Thomas Tuchel felt Neymar's lack of competitive matches showed and he backed Kylian Mbappe after Paris Saint-Germain's loss to Borussia Dortmund.
Neymar was on the scoresheet on his return from a rib injury, while Mbappe also missed PSG's draw with Amiens on Saturday.
Tuchel felt PSG's busy schedule and injuries to their stars had played a part at Signal Iduna Park.
"We have made twice as many matches as Dortmund in recent weeks. We have had injury concerns with key players, they lack the rhythm, the hardness of the competition, as we saw with Neymar," he said, via L'Equipe.
"Our possession of the ball was not stable enough for this not to be a physical match. There was this physical side to this match, in addition, we made too many mistakes in unnecessary positions.
"We were lacking in timing. We lost a little confidence, and form. I thought it would be a more open game."
Neymar said post-match he had been unhappy with PSG's management of his injury, insisting he was ready to return earlier.
Tuchel, who felt his side played with fear, also backed Mbappe to learn from a difficult outing.
"I knew it would be a tough game for him. He had to be very, very patient," he said.
"We know that the technical relationship between him and Ney is good. Ney lacked rhythm, competition.
"Kylian made an assist. It's a tough game for him, a game to grow. I trust him. We're going to have a good game at the Parc [des Princes]."
PSG host the second leg on March 11.