This is a major loss for the last-place Ravens (1-3), who are looking to build momentum after winning their first game of the season Thursday night against the Pittsburgh Steelers in overtime.
Smith has "some small microfractures in his back," according to Harbaugh. The exact number of microfractures is four, league sources told ESPN NFL Insider Ed Werder.
There is hope Smith will play after the Browns game, Harbaugh said. Sources told Werder that pain will be a factor in determining how soon Smith returns.
"I think he's going to be OK in the long run," said Harbaugh, who was interviewed at the Michigan-Maryland game where his brother Jim was coaching.
The Ravens can't afford to lose Smith for an extended period because he's their only consistent playmaker on offense. Smith is Baltimore's leading receiver with 29 catches for 373 yards and two touchdowns. The other Ravens receivers have totaled 25 receptions for 292 yards and one touchdown.
The Ravens' only healthy receivers are Kamar Aiken, Marlon Brown, Darren Waller and Chris Givens, who was traded from St. Louis on Saturday. In addition to declaring Smith out, Baltimore also placed wide receiver Michael Campanaro on injured reserve with a herniated disk.
The Ravens are off Sunday because they played Thursday night. Baltimore plays the Browns at home on Oct. 11 before going on the road to face San Francisco on Oct. 18 and Arizona on Oct. 26.
Smith was injured late in the third quarter on Thursday night, when he was speared in the back by Steelers linebacker Lawrence Timmons. He didn't play in the fourth quarter or overtime. The Ravens initially announced Smith had a lower back contusion.
Earlier Saturday, the Ravens released a statement on Smith, saying his injury is "not a surgical issue." His status was listed as week-to-week.