Michel had already been ruled out for Monday night's game against the Kansas City Chiefs.
Michel, who had nine carries for 117 yards in a Week 3 win over the Las Vegas Raiders, had been limited in practice all last week.
Harris opened the season on injured reserve because of a hand injury. Harris, a 2019 third-round pick from Alabama, began practicing on Wednesday.
Michel had been scheduled to speak with reporters on Thursday, but when that didn't happen, it was an ominous sign about his health after his breakout game against the Raiders that included the two longest runs of his career (48 and 38 yards).
He has battled knee injuries since the Patriots selected him in the first round of the 2018 draft (31st overall) but has missed only three games because of them.
Anticipating Harris' potential return and a possible logjam at running back, coach Bill Belichick said last week, "It's good to have depth. We certainly had times when we haven't had it, so it's a good thing we have it. That's a good problem to have -- having a lot of good football players."
In addition, the Patriots activated receiver/punt returner Gunner Olszewski (foot) off injured reserve, while placing core special-teamer Cody Davis (rib) on IR. The team also elevated practice-squad receiver Isaiah Zuber to the game-day roster as a COVID-19 replacement.