Armstead told NBC Sports Bay Area's Jennifer Lee Chan that he has also been recovering from a hairline fracture in his left ankle.
"I was hoping to be on the earlier side of returning, but I'm still on schedule," Armstead told NBC Sports Bay Area after Monday's light practice, in which the defensive lineman did not participate.
The 49ers are being understandably careful with Armstead's recovery, given the new information. They hope to have the defensive lineman available down the stretch as the team hopes to make a playoff push in the second half of the season.
"Arik's improving," general manager John Lynch said last week. "He had somewhat of a significant deal in that he's coming off an injury with your lower half, and that's hard when you're a guy who is as big as he is. You've got to make sure he's right."
Armstead is also recovering from plantar fasciitis, which, until now, had been reported as being in his right foot. However, Chan reports that the initial setback was in the same foot as the fracture.
Armstead has missed five games this season. In addition to the last four, he missed San Francisco's Week 3 contest against the Denver Broncos.
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