Everton defender Phil Jagielka has told ESPN FC that Manchester United signed the complete centre-forward when they brought Romelu Lukaku to Old Trafford.
Lukaku, who has scored six goals in six games for United since arriving from Goodison Park in July, faces Everton on Sunday for the first time since leaving.
Everton have spent £140 million on new signings, as well as completing a free transfer for former Old Trafford captain Wayne Rooney, but there remains a question mark over their ability to fill the gap left by 24-year-old Lukaku.
Everton captain Jagielka said replacing the Belgium international was never going to be easy and, asked about his strengths, he said: "You normally get somebody who is physical but can't shift -- but once Rom gets going there is no catching him.
"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."
Everton travel to Old Trafford having lost their last three games in all competitions and not scored a goal in 314 minutes of play.
And Jagielka admits that they have not yet found a solution to the problem of how to replace Lukaku's goals.
"That's the million-dollar question," he said. "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."
Jagielka said that if Everton could get "two to three people nicking in with 10-12 goals this season, the loss of Rom's goals will hopefully be dealt with."
Everton's poor start has cast doubts over their ability to challenge for a top-four spot this season, but Jagielka, who has been at the club since 2007, said he believed it was too early to judge their prospects.
"It's been difficult," he added. "We've also had a few European games as well. It's been a tough start.
"Before the Spurs game [a 3-0 defeat at Goodison Park], you'd say it was an OK start, but the Spurs game was disappointing, the manner in which we lost.
"But we can't forget that the two away trips [to Manchester City and Chelsea] have been probably the hardest you could get in the Premier League.
"It's time for us to show our character now. We have a really busy month ahead. The League Cup starts, there are a couple of Europa League trips and Manchester United too, so we need to start picking up some points."
He stressed that there was "no panic" but added: "It would have been nice to pick up more points. But, like most seasons, I think you judge probably after about 10 games. You can't look at four or five games.
"Christmas is crunch time. You get all those games in succession, and hopefully you are at the right end of the league and are pushing up."
Despite the challenge of facing league leaders United away from home, Jagielka said he believed Everton could get a positive result.
"We've had some close encounters with United," he said. "After the performance against Spurs, we need to dust ourselves down in the league and get back to performing a lot better.
"Old Trafford is a tough place, but a great place to play. For me, it's probably the best stadium in the country and one of the best squads that has been assembled, so we can hopefully go there and put a performance in."