The Red Devils completed a deal to sign the Chile international from Arsenal last month, with Henrikh Mkhitaryan heading to The Emirates as part of the same transfer.
Sanchez scored his first goal for the Red Devils when they sealed a 2-0 victory over Huddersfield Town on Saturday at Old Trafford.
Lukaku has struggled to find consistent form for Manchester United this season since signing from Everton in the summer transfer window.
The Belgian, though, has managed to score 12 goals and make five assists in 25 Premier League games for Jose Mourinho’s men.
And Rooney feels that the addition of Sanchez in the January transfer window will help to get the best form from Lukaku in the coming weeks and months.
Speaking on Sky Sports’ Monday Night Football, Rooney said: “I think that’s what Manchester United have lacked – players around Lukaku who are going to push him on as well and try and help him, give him a bit more freedom [for him to] score more goals.”
Manchester United will take on Newcastle United in their next Premier League game at St James’ Park next Sunday afternoon.
The Red Devils currently sit in second place in the Premier League table and 13 points behind leaders Manchester City.
Manchester United will take on Spanish side Sevilla for a place in the Champions League quarter-finals later this month.
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