Faletau was hurt in Bath's European Champions Cup defeat to Toulon, prompting concern for club and country with the Six Nations just eight weeks away.
The back-row missed a chunk of last season and the start of the 2017 Six Nations due to knee ligament injuries.
"I hope it's not the same knee as he injured last season, but it doesn't look good," Blackadder was quoted as saying by the BBC.
"It looked like there was a ruck, he came in from the side and Toby tweaked his medial [ligaments]."
At the very least, Faletau appears likely to miss Bath's reverse fixture against Toulon next weekend.
What. A. Game
Shall we do it all again next week?! pic.twitter.com/wldVzyojKH
— Champions Cup (@ChampionsCup) December 9, 2017
Wales will be hoping Faletau can make a speedy recovery, particularly after fellow back-row forward Sam Warburton was ruled out of the Six Nations earlier this week.
Photo Getty Images. Caption: Bath and Wales forward Toby Faletau