Lukaku faces former club Everton this weekend having netted six goals in as many matches in all competitions for United since his £75million move.
After playing a part in 31 Premier League matches last season, the 28-year-old has only participated in two so far this term including just five minutes against Swansea City.
"But when I arrived at Manchester United, I saw how that had changed".
Spearheaded by Sadio Mane, Koeman's exciting Southampton side picked up two 1-1 draws against a Chelsea side that would go on to win the title comfortably in 2015 while a seventh place finish and European football was just reward for the Saints.
Koeman failed to land an out-and-out replacement for Lukaku in the summer, with Wayne Rooney the only player to have found the net for Everton in their opening four games, and Koeman accepts Dominic Calvert-Lewin and Sandro Ramirez can not be asked to assume Lukaku's role.
United are huge favourites to win the game at 4/11 odds.
"But when the team is in control and makes decisions on the direction of a game, when the team is so offensive, when the team puts so many people in attacking areas, I think they have to score goals".
England left-back Luke Shaw is fit and available but Marcos Rojo will be sidelined for another couple of months as he continues to recover from cruciate ligament damage suffered in April.
"Squads are there to try to cope with these kind of situations".
"We have players waiting for an opportunity", he said. "He just wants to keep improving and scoring goals". Meanwhile, midfielders Ross Barkley (hamstring) and James McCarthy (knee) are likely to be out as well.
After making a shock £75million move to Manchester United this summer, the Belgian global has already begun cementing his name in the club's proud history, having scored five times in his first five appearances for the Red Devils.
"He's come in and already scored a lot of goals for us, so we know what he's about". Ironically, both goals were scored by former United star Wayne Rooney.
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