11:20 PM ET

HOUSTON — The good news for the Carolina Panthers on Thursday night was they improved to 3-0, but the concern is whether they can sustain their success after losing two of their biggest stars in running back Christian McCaffrey and cornerback Jaycee Horn.

McCaffrey suffered a hamstring injury with 12:13 left in the second quarter and did not return. Horn went down with a right foot injury in the third quarter and was carted off. A source told ESPN’s Jeremy Fowler Horn suffered a broken foot and there is no timetable for his return.

Both could miss multiple games.

Quarterback Sam Darnold and the league’s top-ranked defense were enough to hold off the Texans 24-9, but will they be enough when the competition picks up with Dallas on October 3.

McCaffrey entered Thursday’s game as the NFL leader in yards from scrimmage. Almost as valuable as his productivity as a runner and receiver was his blocking.

Darnold wasn’t sacked prior to McCaffrey going down without contact on a run to his left. He was sacked three times the remainder of the second quarter after being replaced by rookie Chuba Hubbard.

Horn’s ability to man-up on the opponent’s top receiver allowed defensive coordinator Phil Snow to attack with more blitzes. That helped Carolina to a league-best 10 sacks the first two games. Whether that will continue against more experienced quarterbacks remains to be seen. They had four sacks Thursday.

Houston was the second of Carolina’s three opponents to start a rookie quarterback.

Horn was replaced by veteran Rashaan Melvin, who was signed to the practice squad on September 14 after being released after training camp.

Buy TE Tommy Tremble: The rookie out of Notre Dame did little in the first two games, but emerged as a weapon in the second half of this one with a rushing touchdown on an end around and a 30-yard catch. He may be evolving into offensive coordinator Joe Brady’s scheme.

Quarterback breakdown: Darnold struggled some immediately after McCaffrey left, in large part because his protection broke down. But he took his game to another level in the second half in a way he hadn’t the first two games. He beat the Texans with his arm and legs, almost willing Carolina to victory and completing 24 of 34 passes for 304 yards. He also rushed for a TD.

Silver lining: After next week’s game against Dallas the Panthers get Philadelphia, Minnesota, the New York Giants and Atlanta.