The 32-year-old was released by the Philadelphia Eagles following a disappointing season in the 2012 campaign and was picked up by Jim Harbaugh's side in March.
"We really didn't now what we were getting when we got Nnamdi," Fangio told CSN Bay Area.
"He's had some good days out here and some days when we weren't sure if he'd be able to still have it. I think we're kind of in between on him right now.
"Hopefully, he'll still be able to have some gas left in his tank to go out there and play like he did prior to going to Philadelphia. So I think the jury is still out there."
Asomugha has been named in the All Pro team in four seasons in his nine-year career.