Peoples-Jones, who led Michigan with 47 receptions for 612 yards and eight touchdowns in 2018, is suffering from a lingering groin injury.
Dwumfour, a starting defensive tackle who had 19 tackles and two sacks in 10 games last season, tore his plantar fascia before Michigan's loss to Florida in the Peach Bowl.
"By all indications it looks like they have more serious injuries than we previously thought," Harbaugh said. "Both have not participated in any practice or all winter. The concern is that [it is] more serious, [and we'll see] if they can get back to their previous form and level."
Harbaugh also said it's possible the injuries could linger into the upcoming season, though neither player is expected to need surgery.
"I've always been, as a coach and when I played, I put a lot of stock in how a player's body feels, how they respond. That's the feedback. All indications we're getting is that they really can't do much at all," Harbaugh said. "You always hope for the best.
"... You start to worry when injuries linger longer than you previously thought, or longer than the time it usually takes. But nobody knows their body better than the actual person who is out there."
Meanwhile, starting wide receiver Nico Collins is progressing after an offseason procedure for an unspecified injury.
Peoples-Jones, who is entering his third season, also had 25 punt returns for 250 yards and one touchdown last season.
ESPN's Tom VanHaaren contributed to this report.