However, Washington told ESPN Dallas 103.3 FM's Eric Nadel that if Darvish is 100 percent healthy, he could start on Friday against the Los Angeles Angels.
Washington had said earlier on Wednesday that it was a "consideration" Darvish would start prior to his scheduled outing Sunday.
The 26-year-old rookie was scratched hours before his Tuesday start because of a stiff neck. Washington said Darvish felt better on Wednesday and received treatment but is not ready to pitch yet.
One reason to start Darvish on Friday would be to line him up for the final game of the season if he's needed. If not, he could pitch a few innings and still be ready to go in Game 1 of the AL Division Series if the club chooses to pitch him in that spot.
"We'll just move the rest of the rotation back if we did that," Washington said on the pregame show on 103.3 FM. "He would have to be 100 percent, not 80 or 90. It would have to be 100 percent."
Washington was not concerned that Darvish's neck would keep him out any extended period and said it was the product of sleeping on it wrong.
"If he's 100 percent before Sunday, you may see him," Washington said. "If not, you certainly will see him Sunday."
Darvish is 16-9 with a 3.90 ERA in 28 starts this season. Tuesday was the second start he's skipped this season because of injury. He missed an outing in late August with a tight quad muscle and has thrown seven straight quality starts since then. His 16 wins are the most by any rookie in the AL. He has 214 strikeouts, third-most in the league.
If Darvish pitches on Sunday he'll be in line to face Zack Greinke.