The Canada striker has struggled in his first six months with Juve.
But Marsch told La Gazzetta dello Sport: "I know Jonathan well, I wasn't worried after the missed penalty against Lecce and the controversy. I saw him live in Pisa, we talk often, I was sure the goal would come soon.
"And not even the hug from everyone surprises me, that's the attitude of the great teams."
Marsch also said: "David is happy at Juventus, even in the toughest moments he always told me he wanted to stay. Now he won't stop and will score a lot, he just needed a period of adaptation.
"After (Erling) Haaland, he's the best striker I've worked with."