Roma have won their last four games to surge level on points with second-placed AC Milan, who ended a difficult week by labouring to a 1-1 home draw with struggling Livorno.
Both sides moved eight points behind leaders Inter Milan, whose match at Parma was postponed because of snow.
Alessandro Del Piero went down under a challenge from Modibo Diakhite and slotted in the 70th-minute penalty before Lazio's Stefano Mauri netted from close range to extend Juve's winless run to five matches.
The little-used Okaka was due to make a loan move to Fulham but was named in Sunday's squad because four Roma forwards were out injured, including Francesco Totti and Luca Toni.
The 20-year-old entered the fray on the hour after John Arne Riise's terrific volley put Roma ahead before Simone Vergassola equalised from a tight angle late in the first half.
"I told him 'you're not leaving now'. He fainted," Ranieri joked to Sky TV, hinting Okaka would still go to Fulham.
Milan could have done with a moment of magic after another lacklustre San Siro display following last Sunday's defeat by nine-man Inter and Wednesday's Italian Cup exit against Udinese.
Before kick-off, chief executive Adriano Galliani declined to deny reports of a swap between Milan left back Marek Jankulovski and Inter winger Mancini on Monday's transfer deadline day.
Livorno, whose coach Serse Cosmi resigned last week before changing his mind, deservedly equalised on 53 minutes when Cristiano Lucarelli prodded in Claudio Bellucci's scuffed shot.
Striker Adrian Mutu has been suspended for testing positive for a banned appetite suppressant and Fiorentina's bid to sign Antonio Cassano from Sampdoria as his replacement fell through.
