Day will miss the third-placed Hawks' clash with Richmond at the MCG on Sunday with a corked calf, as crafty forward Nick Watson makes his return from a hamstring injury.
Mitchell said Day was always going to be managed at some point, with his calf problem simply pressing the issue this week.
The 25-year-old midfielder has played six games this season, having returned in round 12 after undergoing a shoulder reconstruction in January.
Key defender Josh Battle will play against the Tigers after having his appendix removed a fortnight ago, as will Josh Weddle, who returns from suspension.
"Will Day won't play ... he got a cork in his calf and he wasn't going to be able to train today," Mitchell said on Wednesday.
"There were about 14 games between when he came back and the end of the season, and we knew he wouldn't play 14 straight.
"Just one (week) - we knew he had to miss anyway, so this has probably made the decision."
Mitchell said there aren't any plans to manage Day again before finals.
"I don't think so. I mean, it depends what happens," he said.
Elsewhere, Collingwood have ruled out Jeremy Howe as a precaution after the key defender pulled up with hip and groin soreness following last week's win over North Melbourne.
Lachie Schultz also hurt his ankle against the Kangaroos and could miss up to four weeks with a low-grade syndesmosis injury.
Fellow defender Isaac Quaynor (ankle) has proven fit to face Carlton at the MCG on Saturday night, with Harvey Harrison and Charlie West also returning to the side.
"It's good to have Q back," Magpies coach Craig McRae said.
"He was close last week, and Howey's got a minor thing, and he should be available with a five-day break coming.
"We could argue we push him to the line, but then he may be at risk of further injury, so we'll leave him out this week.
"It's just a bit of wear and tear at this time of the year for a guy that's in his 16th season."
For the Blues, George Hewett was named on the bench despite a back injury, while Jacob Weitering (calf) remains on target to return in round 23.
Meanwhile, Fremantle star Shai Bolton has been rested for the ladder leaders' meeting with Port Adelaide on Saturday.
Dockers coach Justin Longmuir wouldn't shed light on midfielder Bolton's health, but the 27-year-old has been managing a hand issue.
Hayden Young returns from a groin injury to slot in for Bolton, with Mason Cox retaining his ruck spot ahead of Sean Darcy.
Key forward Todd Marshall has been recalled among five changes for Port Adelaide, with star quartet Zak Butters (ankle), Jason Horne-Francis (suspended), Jack Lukosius (calf) and Lachie Jones (ribs) unavailable.
Brisbane's Bruce Reville is out due to personal reasons, replaced by James Tunstill for Saturday night's Gabba clash with an unchanged West Coast.
Ruck star Tim English, who missed last week with a back issue, has been named in an extended Western Bulldogs squad for Sunday's clash with Gold Coast.
6°C