A source told ESPN's Adam Schefter on Tuesday that Woodhead's recovery time will be closer to six to eight weeks.
Woodhead, who signed a three-year, $8.8 million contract (including $4.25 million guaranteed) this offseason with the Ravens, was injured on the team's opening drive of the season Sunday. The 32-year-old running back fell to the ground holding his hamstring and had to be carted to the locker room.
"Danny looks like it's going to be a while," Harbaugh said. "It's a hamstring. It's a little unpredictable."
The Ravens could decide to place Woodhead on injured reserve and bring him back around midseason as one of their two designated-for-return players.
Harbaugh wouldn't rule out adding a running back from another team. Baltimore can also promote one of the two running backs from the practice squad: Jeremy Langford or Alex Collins.
"Those two guys have done a really good job at studying the offense and they're healthy," Harbaugh said. "Those are two very good options for us. We'll see by the end of the week where we're at running back."
The Ravens were hoping Woodhead was going to be a catalyst for the NFL's No. 21 scoring offense from a year ago, especially based on his only drive with Baltimore. In six snaps, he caught three passes for 33 yards (picking up first downs on each one) and ran the ball once for 4 yards.
This isn't the first time Woodhead has dealt with this injury this year. He was sidelined for the last two preseason games because of an issue with his hamstring.
Staying healthy has been a problem for Woodhead. He has missed 27 games over the previous three seasons. A torn ACL ended his season last year with the Chargers after two weeks.