Lammens is in line to be between the posts for United's clash with Manchester City - in his first appearance for the club.
Stepney told The Sun: "Senne, like all youngsters in his position, needs to show he has the courage to be his own man.
“So many these days don’t grasp that the 18-yard box — and especially the area from the six-yard line to the penalty spot, is theirs to command, no messing.
“It is their territory and they have to command it and show they have courage but also authority to do so. If they do that they will get the instant respect of team-mates and opponents.
“Some might say throwing Senne into the derby after growing up in Belgian football is too much pressure too soon.
“But it could be the perfect opportunity to show right away that he has what it takes to handle the huge responsibility of playing in net for Manchester United.”
Desire and courage
Stepney made his United debut against City 59 years ago.
He recalled: “I well remember that debut of mine after being signed from Chelsea for £55,000 — a world record for a keeper back in 1966. City were really good and had top players like Mike Summerbee, Colin Bell and Francis Lee.
“Not long into the game at Old Trafford I claimed my first cross, Summerbee delivered it, climbing up through a ruck of players and right away they all saw that I had command of my box.
“That gave me confidence, my team-mates confidence and while — like all of us in the goalkeepers’ union — I would have my dodgy moments, it showed I was up for it and we won 1-0.
“Senne has to show that same desire — like Donnarumma will have to do at City — and prove he has the inner strength that all top keepers have.
“You not only have to show you have character, you have to be a character, your own man, not some over-coached robot afraid to show his personality.”