Sanchez joined United from Arsenal in January as part of the deal that saw Henrikh Mkhitaryan move to the Emirates Stadium.
The 29-year-old has struggled to make his mark at Old Trafford - managing just one goal in all competitions, with his performances for the Red Devils constantly under the microscope.
Lukaku, however, has insisted that Sanchez will "come good" for United as he is "a quality player with a lot of assets".
"As long as he keeps working hard like he does, I think he'll come good for us," Lukaku told reporters. "He is a quality player with a lot of assets.
"Look at my situation. I started really well and then had the dry spell in November and December. But at the end of the day with hard work you are always going to get through it."
Next up for United is a home Premier League match against Swansea City on March 31.