Bell has held out since training camp after failing to come to terms on a contract extension with the Steelers. He has yet to sign his franchise tender.
Conner put up big numbers during Bell's absence Sunday, rushing 31 times for 135 yards and two touchdowns while catching five passes for 57 yards.
The decision to start Conner came after Bell failed to appear at team meetings and report to the team last week.
On Monday, the Steelers removed Bell from their depth chart, which suggests they may have suspected he didn't intend to report this week either.
Last week, several Steelers players spoke out against Bell's decision to continue holding out, including center Maurkice Pouncey, per ESPN.com's Jeremy Fowler:
"If you don't want to be here, it is what it is. Hold out 10 weeks. ... I just felt confident that he was gonna come, but now that he didn't, obviously it's Le'Veon over the Steelers, and we're the Steelers and we're going to play as the Steelers. ... Now when it's game time and you that you have $14 million looming out there, and you're still not here and your team really wants you here? At this point, we got Conner."
While Bell isn't getting paid during his absence, he can miss as many as 10 weeks and still become a free agent during the offseason if he reports to the Steelers by Week 11.
Bell forfeited an $853,000 game check in Week 1, and he will miss out on the same amount if he doesn't report for Week 2.
Although Conner has promise, Bell is a three-time Pro Bowler and two-time All-Pro first-team selection who racked up 1,946 yards from scrimmage and 11 touchdowns last season.
Without him, the Steelers would be missing a significant part of their offense against a Chiefs team that scored a big road win over the Los Angeles Chargers in Week 1.