Mahomes, who sustained a dislocated kneecap during the Chiefs' win over the Denver Broncos last week, was ruled out for Kansas City's game against the Packers, according to the team's official injury report.
"He just wasn't ready right now," Reid said. "That was our decision, not his. He wanted to play right after it happened. He just needs a little bit of time here."
When Mahomes suffered the dislocated right kneecap, a member of the Chiefs' training staff appeared to pop his knee into place. The initial belief was the reigning NFL MVP would miss at least three weeks due to the injury.
The team received positive news after an MRI, and Mahomes was able to participate on a limited basis. Earlier in the week, Reid wouldn't rule his signal-caller out and expressed optimism about the injury.
"It was the best possible result, and I just kind of program in the way he rolls," Reid said. "[Athletic trainer Rick Burkholder] does a good job of managing those [injured] guys so they're not doing too much or not enough. ... I trust them to do their job.
"After it happened, [Mahomes] thought he could go back in. That's how he's wired. He's wired a little different that way. ... He attacks everything. That's how he goes about it. He's been full steam ahead."
Despite the positive outlook, Mahomes is out and backup quarterback Matt Moore will make his first start since Week 11 of the 2017 season when he played for the Miami Dolphins. Undrafted rookie Kyle Shurmur will be the backup to Moore.
Through seven games this season, Mahomes has completed 157-of-241 passes for a league-best 2,180 yards, with 15 touchdowns and one interception.