The Belgium international left Everton and completed a switch to Old Trafford in the summer transfer window. He scored on debut in the Premier League and Champions League for United and has a tally of 11 goals in 10 matches in all competitions, including seven in the Premier League.
Lukaku is currently the Premier League's leading goalscorer after seven matches. So far, the 24-year-old has failed to score in just one match in all competitions - during the Red Devils' 2-0 league win over Leicester City.
Aguero, on the other hand, has seven goals in all competitions, which include six goals in the league. Robson said Lukaku's technique is not "great", but the former Chelsea striker's ability to score goals has made the ex-English midfielder choose the United frontman over the Argentine international.
"At the moment, yes (I would have Lukaku over Aguero). At the moment I wouldn't have Aguero in there," Robson told ESPN FC.
"Aguero's a great player but if you look at Harry Kane, I think he's the all-round centre-forward. I think Lukaku's technique isn't great but he's scoring goals and is always a threat when the ball comes into the box.
"If I was the manager at the moment, the way that Lukaku can score goals out of nothing, the way he can run in behind, the way he can win balls in the box.
"Sergio Aguero, I think he's a great player, does he work hard enough? I'd still want people to close the ball down and do the things that Pep Guardiola wants him to do, which he's not always doing. So I would still go with Lukaku."