Miami Dolphins quarterback Ryan Tannehill has been ruled out for Sunday's playoff game at Pittsburgh, the team announced Thursday.

With Tannehill out, backup Matt Moore will start for the fourth week in a row.

"He knows he's our relief pitcher,'' offensive coordinator Clyde Christensen said of Moore. "We try to make the decision early in the week, because it's hard to get two guys prepared. He assumed he was doing it. He has continued to prepare as a starter.''

Prior to Thursday's practice, the Dolphins determined Tannehill wasn't ready to return from the sprained ACL and MCL in his left knee, which he suffered in Week 14.

"Yesterday, just watching him move around, I felt like we still need more time," coach Adam Gase said. "I felt like it was the right thing to do, just so we can move on this week and focus on what we're doing right now with Matt.

"There's just no gray area for anybody. I think this week is very important for us to focus on the task at hand and not have anything that can possibly distract your guys."

Ryan Tannehill won't play in Sunday's playoff game. The Dolphins will go with Matt Moore as their starting quarterback for their first playoff appearance since 2008. Steve Mitchell/USA TODAY Sports

Moore, a 10th-year pro, is 2-1 as Tannehill's replacement and will make his first career postseason start. Moore has eight touchdown passes, three interceptions and a quarterback rating of 105.6, which is higher than Tannehill's career-best 93.5 this year.

"He's been in the league for more time than Tannehill, so he's got more experience,'' Steelers cornerback Ross Cockrell said of Moore. "For us on the back end, whether it's Matt Moore, whether it's Tannehill, it doesn't make a difference for our preparation. Our job is still to stop the run, create turnovers and put pressure on the quarterback.''

Tannehill hadn't missed a game in five NFL seasons until he was sidelined against the Arizona Cardinals. He has never played in the postseason, and the playoff game will be Miami's first since 2008.

ESPN's James Walker and Adam Schefter and The Associated Press contributed to this report.