Mourinho re-signed the 35-year-old late in August and Lukaku, who joined United from Everton earlier in the summer, knew that Ibrahimovic had already made a decision to rejoin the Old Trafford club.
When asked if he believes Lukaku is good enough, Van Nistelrooy added: "Why not?"More news: Are Analysts Bullish Workspace Group plc (LON:WKP) After Last Week?
Lukaku still has a long way to go to reach the 150 strikes Van Nistelrooy netted for United, but the Dutchman likes what he sees.
"Henrikh Mkhitaryan has had a great start to the season in terms of assists for the same reason".More news: Alabama vs. Florida State
Ruud van Nistelrooy scored as a Manchester United legends team beat their Barcelona counterparts 5-3 on aggregate after a 2-2 draw at Old Trafford.
Sadio Mane (three goals) and Mohamed Salah (two goals plus one assist), who were both impressive in Liverpool' last outing against Arsenal, are the Reds - one of seven teams now unbeaten - representatives. "Obviously for the young players he is a big personality who they can look up to and learn a lot of things".
United are next in action at Stoke on Saturday, while the club kicks off their Champions League campaign Tuesday against Basel, before taking on Rooney and Everton the following Sunday.More news: After Losing In Court, Florida Anti-Death Penalty Prosecutor Charts Way Forward