The Texans activated Taylor's 21-day window on Wednesday, when he returned to practice after missing five games on injured reserve with a left hamstring injury.
"We just don't feel Tyrod [Taylor] is ready right now," Culley said. "He's made a lot of progress, but not enough progress to be ready to get back out there and start."
Taylor was injured during the first half of the Week 2 game against Cleveland. He was replaced in the second half of that game by Mills, a third-round pick in this year's draft.
Mills has completed 63.3% of his passes for 912 yards, with five touchdowns and seven interceptions this season. The Texans have not scored a touchdown on the road since Week 2 and have been outscored 102-8 in their past three road games.
Culley has said multiple times that Taylor will be the Texans' starter when healthy and that he doesn't believe in players losing their jobs because of an injury.
"[Taylor] gives us the best chance to win," Culley said earlier in the month. "He's our starting quarterback. ... He was doing an excellent job, and when he comes back, he will still be our starting quarterback."
Before he was hurt in Week 2, Taylor had completed 70.5% of his passes for 416 yards, with three touchdowns and no interceptions.
After the Texans' Week 7 loss to the Cardinals, Culley said he wanted to "make sure that when [Taylor is] running around back there that there's nothing hampering him from being the kind of player that he is, using his legs."