Mbappe suffered an ankle injury during the Coupe de France final against Saint-Etienne on July 24, with the club revealing the "serious" sprain would likely keep the 21-year-old out for three weeks.
The France international watched on as PSG completed a domestic clean sweep on Friday, securing victory in the last ever Coupe de la Ligue final via a penalty shoot-out against Lyon.
Thomas Tuchel had said on the eve of the match that Mbappe requires a "miracle" to be ready for the quarter-final tie with Atalanta, which takes place in Lisbon on August 12.
Herrera, however, is more upbeat over the forward's chances, though is confident the squad can still prosper without him.
"We are optimistic with Kylian. We have seen that he can walk quietly," the former Manchester United midfielder said after the game with Lyon.
"He's very professional, he's young. He really wants to play and we can be very calm. He works every day, morning and afternoon. But if it's not possible for him to play, we also have other solutions.
"Kylian is very important and I'm sure he will try to play."
PSG are next in action on Wednesday, taking on Sochaux in a friendly prior to resuming their European campaign.
"It's a fantastic week for us," Herrera said after completing a dominant domestic campaign. "We won two titles and we are very, very happy.
"A lot of players have played and participated in this success - I can say that we are ready for the Champions League now."