Neymar has quickly put an off-season transfer saga behind him, scoring two late winners in as many games, including in Monday's (AEST) 1-0 victory at Lyon.
Tuchel said the Brazil forward will improve at PSG, who hold a three-point lead atop the Ligue 1 table.
"He can still do better. It's only his second or third game in four months," the PSG head coach said.
"He can do better and he will do better because he will gain in a physical capacity, but it's Ney, he's always decisive. He always thinks to play forward, to attack.
"We need players like that in tough games like this to make a difference in one-on-ones."
Neymar was heavily linked with a return to LaLiga champion Barcelona during the close season, but ended up staying in Paris.
Tuchel said the 27-year-old forward was committed to PSG despite wanting the off-season move to Barca.
"He has a contract so of course he is 100 per cent with us," he said. "I can even confirm that he is 100 per cent involved with us on the field and 100 per cent in the group.
"He laughs a lot with his team-mates, it's normal for him and it's good for the team."
Tuchel said: "He is here to score, he is there to do decisive things so I am very demanding with him."