Lazio, who had won all four previous games, were held to a frustrating scoreless draw by a Bulgarian side who came to defend.
Second-tier Forest's buoyant fans were briefly silenced when Leicester grabbed a lifeline before halftime as Kelechi Iheanacho pounced on an error to score from an acute angle.
Ibrahimovic was forced off with a muscle problem with 11 minutes remaining, and the Swede was pictured sitting on the bench with an ice pack strapped to the back of his left leg.