Monday evening’s 1-1 draw with Sassuolo.
The visitors went ahead through Borja Valero, but Sergio Floccari leveled the score to frustrate La Viola.
“I’m sorry, we have paid for our midweek Europa League fixture as we were tired,” Sousa told Sky Sport Italia.
“We looked better in the second half, but struggled towards the end of the game. We all worked very hard.”
Giuseppe Rossi was in the starting-eleven, but Sousa believes the striker didn’t get the service he needed.
“Rossi is working to get back to a high level, he needed better service so that he could play,” he added.
“It is important that he is allowed to play, but he does need help from his teammates.”
Fiorentina missed the chance to go top of the Serie A table and remain in third position.
