PITTSBURGH -- Despite preparing for quarterback Justin Fields to start a fourth consecutive game for the undefeated Steelers on Sunday against the Indianapolis Colts, coach Mike Tomlin declined to name him the starter going forward over veteran Russell Wilson, who has been sidelined with a lingering calf injury.

"Because there's no need," Tomlin said when asked why he won't name Fields the permanent starting quarterback. "I explained to you the variables of the week. It has not changed. He's going to walk in this building with that mindset tomorrow, and so really, there's no need to. Sometimes in this business, man, there's a myriad of complex decisions that need to be made.

"I've learned to make them when it's appropriate and it's not necessary as we sit here right now, when Russ gets to an appropriate point of health and we have a decision to make, I'll make it and I'll announce it and I'll be really transparent about it, but until then, I don't care how many ways you guys ask me, I got no intentions of making the decision that's unnecessary at this juncture."

Tomlin, though, was vague when asked to elaborate on what an appropriate level of health is for Wilson, who initially injured his calf in late July on the eve of training camp during the team's conditioning test.

"We'll know when we get there," Tomlin said. "He'll not only be healthy in terms of keeping himself safe, but he'll be healthy in terms of being able to be productive and do the things that he needs to do and we need him to do."

As has been the case for the past two weeks, Wilson will be a limited participant in Wednesday's practice. But Tomlin said they'll revisit Wilson's status at the end of the week if his practice participation increases throughout the week. Wilson has been inactive and served as the team's emergency third quarterback for each game this season.

"As we walk in the building tomorrow, I want Justin singularly focused on starting for us and doing what's required to be ready to do that," Tomlin said. "... We'll see where the week leads us. If [Wilson's] availability or the quality of his practice participation picks up, maybe we'll consider that later in the week. If it doesn't, then it won't be a consideration, and we'll just continue with where we are right now."

Tomlin initially named Wilson the starting quarterback after an open training camp competition, though Wilson was given "pole position" for the job soon after signing a one-year contract with the Steelers in free agency. He was poised to start in the season opener in Atlanta until he aggravated his training camp calf injury on the Thursday of Week 1. Since then, Fields has started in Wilson's place and helped the team to a 3-0 start, capped with his best performance of the season in the win against the Los Angeles Chargers. Fields completed 25 of 32 passes for 245 yards with two touchdowns (one rushing, one passing) and an interception in the 20-10 win.

While Tomlin acknowledged that Fields has played well in his three-game stint, including just one turnover, he said that the former first-round pick's performance hasn't factored into his decision-making around a long-term starter at the position.

"Justin has played well, and that's exciting for him and for us, but I'm not surprised by that," Tomlin said. "And I hadn't looked at it in terms of the decision-making because it's not time yet." And while Wilson continues to miss time, Tomlin acknowledged that concerns about his ability to jump into a new system after the injury are less significant because Wilson has been in the league for more than a decade. But asked if there's a point when Wilson might not be able to jump on the proverbial train because it's moving too fast, Tomlin deflected.

"I don't know, man," he said. "We'll know when we get there."