SANTA CLARA, Calif.—Mike Singletary radiated an uncommon intensity during his Hall of Fame career with the Chicago Bears. He then spent a decade giving motivational speeches before getting into coaching in 2003 as Mike Nolan’s right-hand man.

With yet another season nearly lost in San Francisco, the woebegone 49ers will need every bit of intensity and motivation they can get from the former linebacker with the famously fierce eyes.

Singletary became the 49ers’ interim head coach Tuesday, replacing his coaching mentor with a vow to build on the foundation he built with N...

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