Haralson (center) was placed on IR with an undisclosed arm injury

The 49ers announced the corresponding roster to move to Sunday’s signing of OLB Clark Haggans: Parys Haralson, the team’s only back-up OLB last season, has been placed on injured reserved with a left arm injury.

Haralson received the injury during the August 26th preseason game against the Denver Broncos, per Cam Inman of the San Jose Mercury News. According to Inman, “Haralson appeared to get hurt 10 1/2 minutes before halftime.”

“Haralson lined up on the right edge and engaged in a block with Broncos tight end Joel Dreesen,” Inman writes. “Knowshon Moreno cut inside away from Haralson, who was standing toe-to-toe with Dreesen when he suddenly looked at his left elbow (or bicep, more so than his wrist) and headed for the 49ers’ sideline.”

Though Haralson left much to be desired as a pass rusher, his loss is a huge blow for the 49ers’ run defense. Haralson stopped 83% of runs to his zone last season, holding opponents to only 1.7 yards-per-carry. His presumed replacements, Aldon Smith and the newly signed Haggans, had success rates of 74% and 63% respectively. Opponents averaged 6.1 yards-per-carry against Smith and 3.3 against Haggans. Clearly, neither are the run defenders that Haralson is. (Note: Smith only saw 19 runs plays come his direction las season, so the sample size is really too small to draw any definitive conclusions).

In any case, the loss of Haralson leaves the 49ers dangerously thin at OLB, as Haggans could be disciplined by the league for his DUI arrest last April. If the team needs immediate depth, they could look to practice-squader Cam Johnson or the recently released Eric Bakhtiari, who lead the 49ers in sacks this preseason.