Sellers, who is suffering from a bulging disk, received an epidural shot for the pain, but he still hasn't participated in any baseball activity.
"I don't think we're on track for 15 with Justin," Mattingly said.
As for Ted Lilly and Mark Ellis, Mattingly said the doctor is expected to evaluate them Wednesday night, and then the team will figure out the next step in bringing them back.
He added he hasn't heard a timetable on Juan Uribe, but he said the infielder is going in the right direction.
The manager also said Juan Rivera is still on pace to return Friday if he's feeling good after two games with Class-A Rancho Cucamonga on Wednesday and Thursday. Rivera went 0-for-3 while playing left field and first base in seven innings Wednesday.