The forward hasn't played since sustaining the injury in the third period of a 4-1 loss at the Toronto Maple Leafs on Dec. 21 when he was hit to the ice by defenseman Jake Muzzin.
At the time of the injury, Blashill said Mantha, who also missed eight games earlier this season with a knee injury, would be out a minimum of four weeks.
"He's still a ways away," Blashill told the Red Wings website. "A month, maybe. It's not a progress thing. ... It just is what it is. It's going to be a long process before he's healed."
Mantha ranked second on the Red Wings in goals (12) and points (24) prior to sustaining the injury. He scored an NHL career-high 25 goals in 67 games last season, missing a month because of a broken hand.
Mantha has 159 points (80 goals, 79 assists) in 246 games in five NHL seasons, all with Detroit.
The Red Wings, who visit the Minnesota Wild on Wednesday (8 p.m. ET; NBCSN), will be without defenseman Mike Green and forward Frans Nielsen, each because of an upper-body injury sustained in a 6-3 loss to the Colorado Avalanche on Monday. Green will be out a few weeks, Blashill said.
