Everton defender John Stones has faced criticism for the first time in his career recently.
Stones has made a series of high-profile errors in recent times for the Toffees, and Lukaku believes that the pressure he is under from Ramiro Funes Mori is part of the reason for this.
Funes Mori only arrived at Everton last summer, but he has impressed whenever Roberto Martinez has put him into the first-team.
And Lukaku said: “Stonesy is going through a period that he has never had in his career.
“He is going through a period now because he knows Funes Mori is knocking on the door and Jags is doing great.
“That is when you know it is a mental aspect now that has to come up.”
Stones has faced criticism for the first time in his career over the past few months, with many suggesting that the 21-year-old has regressed.
The youngster’s composed defensive style has come under scrutiny, but Everton’s backline has actually tightened up of late.
The Toffees have kept three consecutive shut-outs for the first time this season in their last three outings.
Everton’s good run of form has seen them progress to the FA Cup fifth round, and also climb up to eighth in the Premier League table - and belief is rising that Martinez’s men could have a productive end to the season.