The 24-year-old comes up against his former employers, where he spent four year of his career, for the first time on Sunday afternoon when the Toffees visit Old Trafford.
Jagielka knows all about Lukaku's goalscoring prowess, seeing him net 87 times in his 166 games for Everton, and the Englishman insists that replacing his goals in the side was never going to be straightforward.
"You normally get somebody who is physical but can't shift - but once Rom gets going there is no catching him," he told ESPN. "He is still a young man, but he is just about coming to the part of his career when you expect him to kick on. Hopefully that's not the case at Old Trafford on Sunday, but we expect him to be one of their key players. We knew Rom was going to go on to great things, and he has started his career at United fantastically well.
"He is a difficult player [to mark]. We have trained against him for quite a few years and played alongside him and, at the moment, we are obviously missing his goals. But the way football is, and the crazy way the market is, Rom left us. It's the way it goes, but when you have played with and alongside somebody for a while, there is a friendship there and a battle to want to come out on top. The only way we can do that is stopping him scoring. It's obviously vice-versa for him.
"How do we replace him? That's the million-dollar question. As soon as we knew that Rom would be on his way - and Ross [Barkley] has also not figured this season, so we have lost a few goals - we had to spread goals around the team and it's all about everyone else now. Gylfi [Sigurdsson] has come in, and hopefully if he can't score goals he can assist a high number.
"It's for the other boys now. Dominic [Calvert-Lewin] has come into the team after his great summer, Sandro has come in but not hit the ground running as far as goals go, but hopefully once we get through this tough period we will find some more fluidity further up the pitch."
Lukaku has scored six goals in six games for United since arriving in a £75m deal, including in three of his four Premier League outings.