England and Wasps flanker Jack Willis could be sidelined for the rest of the season but is yet to learn the full extent of the knee injury sustained in Saturday’s win against Italy.
Willis’ left leg buckled awkwardly after Sebastian Negri pulled him out of a ruck, leaving him crying out in pain.
The Wasps back row will see a specialist in the next fortnight.
“It is a pretty significant injury,” said Blackett.
“We will know a lot more when he sees the specialist and it settles down. I can’t tell you exactly how long [he will be out].”
Italy flanker Negri has since apologised for the tackle and wished Willis “all the best”.
Asked if Willis would be out for the season, Blackett added: “I think he is going to be out for a good chunk of time. I don’t want to say exactly, but I think you are not a million miles off that timescale.”
On Monday, Willis wrote on Twitter that he was “truly gutted” but that he would work “to come back a better and stronger player”.
The 24-year-old won the Premiership player of the season award last season and broke into the England team in the autumn having battled back from previous knee and ankle issues.
Blackett added: “He is pretty down, probably because when you have had a serious injury you know what is coming and you know what a big challenge it is.
“The one thing about Jack is he is an out-and-out professional, and he will get back as quickly as anyone could possibly get back.”