UPDATE (5:33 p.m. ET): Allen returned to the game in the second quarter and caught his second touchdown of the day.
His availability for the remainder of the game is unknown.
Allen went down late in the first quarter after catching a 12-yard pass from Philip Rivers. The rookie receiver landed hard on his shoulder, though he did not take a hit from a defender on the play. He jumped to his feet and took several steps before sinking to his knees in pain. He was seen heading into the locker room with medical staff members and has yet to return to the field.
Allen is the difference in the game right now, having caught a 43-yard touchdown pass to give his team a 7-0 lead. He had two receptions for 55 yards before leaving.
The third-round pick out of California has impressed this season, catching 60 passes for 898 yards and four touchdowns.