Ljajic passed a late fitness test before being introduced in a game-changing switch on 51 minutes by Roma boss Rudi Garcia and the former Fiorentina man was influential in a victory over 10-man Lazio.
Full-back Federico Balzaretti burst into tears after slamming home his first goal of the season on 62 minutes, moments after smashing the post with another terrific effort.
Roma were in full control throughout and Ljajic won and converted an injury-time penalty to add some polish to the victory.
But it was from the explosive and free-scoring encounter we’ve come to expect, with Lazio holding the upper-hand over their rivals in recent seasons.
The Lazio fans missed the kick-off as part of a protest against the club’s lack of ambition in the transfer market.
Yet they didn’t miss much and their arrival, met by jeers from Roma fans and explosions of pyrotechnics, was the early highlight.
Some bright play from Antonio Candreva in the wide-areas was the most likely route to goal for Lazio with the Italy international winning some free-kicks around the box.
But Garcia’s side were within inches of netting after half an hour when Francesco Totti picked out former Arsenal man Gervinho at the back-post from a free-kick and the Ivorian glanced his header wide.
The interval broke up a scrappy end to the first 45 minutes with four yellow cards awarded in the space of 15 minutes.
The decision to introduce Ljajic into the action six minutes after the restart completely changed the tempo and purpose of the game.
Less than a minute after his arrival, the Serbian international danced past a few Lazio defenders and forced Michael Ciani into a dangerous clearance towards his own goal.
Totti’s resulting corner-kick was met by Manchester United target Daniele de Rossi who headed inches past the post.
But it was only a matter of time before the league leaders went in front with Totti setting up Balzaretti, who smashed a superb left-foot strike against the post before the same combination finally opened the scoring for Roma seconds later.
Lazio were stretched towards the last 10 minutes and Totti was cynically fouled 30 yards out by substitute Andre Dias, who was shown a straight red-card by the referee.
Ljajic came close from the free-kick, forcing Marchetti into a decent save under the crossbar – but the young attacker would soon get his joy in added time.
After Marco Borriello passed up a golden opportunity, Ljajic weaved clear of Lazio defender Lorik Cana and he was fouled with the referee awarding a spot-kick.
The Serbian stepped up coolly, showing great maturity, to send Marchetti the wrong way and secure the three points for Roma.