Ole Gunnar Solskjaer extended his winning start at Manchester United to eight games in all competitions as Brighton were beaten 2-1 at Old Trafford.
The hosts were dealt a blow prematch when Luke Shaw pulled out through illness and was replaced in the XI by Diogo Dalot.
United took the lead on 27 minutes from the penalty spot, Paul Pogba converting after he had been fouled by Gaetan Bong.
Ten minutes later Anthony Martial almost added a second after a ball over the top from Victor Lindelof but he was denied by Brighton goalkeeper David Button.
But, three minutes before the break, Marcus Rashford scored his fifth goal in his six Premier League games since Solskjaer took charge to all but make the points safe for United.
On his 150th appearance for the club the 21-year-old curled one into the top corner from a tight angle.
Gross reduced the arrears out of the blue in the 72nd minute, drilling in a close-range shot off the underside of the bar after Davy Propper floated in an inch-perfect cross with the outside of his foot.
United, who missed several chances to increase their lead in the opening hour, were forced to hang on in the closing stages as the visitors pounded a flurry of crosses into the penalty area.