Lukaku has made a flying start at Manchester United since joining the Red Devils from Everton back in July.
The Belgian forward has scored six goals in six Premier League games to help Manchester United stay joint top of the table along with Manchester City.
Lukaku was the match-winner once again as he scored the only goal of the game in Manchester United’s 1-0 win over Southampton at St Mary’s on Saturday afternoon.
However, former Liverpool captain Gerrard believes that Lukaku will face a fresh and testing challenge when Ibrahimovic is back available for selection following his lay-off with a knee injury.
Speaking to BT Sport on Saturday night, Gerrard said: “It will be interesting to see how he reacts when Ibrahimovic is back fit and is breathing down his neck for a starting position. Can he cope with that pressure?
“I’m not sure Ibrahimovic is going to be a good sub and he will be putting pressure on to start games.”
Ibrahimovic is expected to make his return to action for Manchester United in the new year.
Lukaku will be hoping to start for Manchester United when the Red Devils return to Champions League action on Wednesday night with a crunch clash against CSKA Moscow in Russia.
The Red Devils kicked off their Champions League campaign with a 3-0 win over Basel at Old Trafford earlier this month.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news