The Argentinian suffered the injury to the hamstring in his right leg in training on Friday and was sent for tests on Monday.
And Valencia have revealed that Piatta, signed from Almeria in 2011, is set for another spell on the sidelines after also suffering a thigh problem back in September.
A statement on the Valencia website read: "Tests on Monday morning confirmed that the player (Piatti) has a hamstring injury in his right leg, with a recovery time of 4-5 weeks."
Piatti will now miss both legs of Valencia's UEFA Europa League last 32 tie with Dynamo Kiev, the first of which takes place on Thursday.
The 24-year-old have scored five goals in 18 appearances, with his most recent strike coming in a surprise 3-2 win at La Liga leaders Barcelona at the start of this month.
Valencia also confirmed that on-loan midfielder Oriol Romeu has begun his recovery in London after successfully undergoing surgery last Monday to repair torn cartilage in his knee.
Meanwhile, defender Philippe Senderos has joined Piatti and Romeu on the injury list after scans revealed that the former Arsenal man has sustained a groin strain.
