During the 2-2 draw with Strasbourg, Eric Choupo-Moting had a golden opportunity to secure the points needed, but his effort from close range went agonisingly wide.
They only dropped points in three of the 23 games prior to the aforementioned games, where they, surprisingly, lost twice and drawn once.
"Before our last three league games, we had a 20-point lead and everyone was saying the league was over, but that wasn't the case,” Tuchel said.
“Against Strasbourg, Lille (1-5) and Nantes (2-3), we didn't manage to wrap up the title, and there are a lot of reasons for that. Now we have to show that we're able to stop this situation and win the title.
“It's not the moment to focus on Lille's result, but focus on ourselves. I expect a reaction from us against Monaco, a good performance, and that we show an attitude that matches our ambitions.
“On Sunday, it'll be vital to show our hunger to win and that we can play as a team. That's the only challenge we have in mind."
Alongside some senior members, including long-term absentee Neymar, Choupo-Moting is yet to be assured of a place in the squad for Sunday's crucial clash due to injury.
"Juan Bernat is suspended while Thomas Meunier and Thiago Silva are both injured, so they won't be with us either," Tuchel added.
“For Thiago Silva, it's possible he returns for the Coupe de France final. Stanley N'Soki will play with the reserve team this weekend.
“Regarding Neymar Jr, Angel Di Maria, Marquinhos, Edinson Cavani, Julian Draxler, Eric-Maxim Choupo-Moting and Christopher Nkunku, there are still two training sessions - one on Saturday and then Sunday morning - to decide.
“It's possible they join the squad, but a decision will be made after those last two sessions. They are absolutely key players, for the qualities and their attitude.
“Our challenge is to have a strong squad for two crucial games, against Monaco and then Rennes in the Coupe de France final."
If PSG wrap up the Ligue 1 title on Sunday, it would be the first time Choupo-Moting won a trophy in his senior career.
On the international scene, his Cameroon side have been placed into Group F at the Africa Cup of Nations, which kicks off on June 21 in Egypt.
The Indomitable Lions will begin their Afcon campaign against Guinea-Bissau in Ismalia, before meeting Ghana and Benin as they look to reach the knockout stages.