Harris's surprise resignation will almost certainly leave the club without a permanent manager as title contenders Leeds visit The Den on Saturday afternoon.
Millwall had been on a run of eight games in all competitions without a win, but the London club were still five points clear of the relegation zone with 11 points from their first 10 Championship games.
Only three of those eight winless games had seen them beaten in 90 minutes, and there was little pressure on Harris, meaning he may have stepped down with an eye on a potential move to another club.
On the contrary, Marcelo Bielsa's project at Leeds is well underway and the Yorkshire club have been dominant in games so far this term.
However, they have often struggled to kill off games despite their possession and chances, dropping points on a few occasions due to their profligacy.
They did manage to claim an important win in their last game, though, as Ezgjan Alioski's solitary strike ended West Brom's unbeaten league run.
The last meeting between Leeds and Millwall was at Elland Road and saw the former emerge 3-2 winners in a thrilling encounter.
Millwall have only lost one game at home so far this season, however, and will be difficult to beat on their own patch.
Millwall form: DDLLDD
Leeds form: WLWDLW
Millwall have no fresh fitness issues to contend with and can pick from a full complement.
Millwall possible starting lineup:
Bialkowski; Romeo, Hutchinson, Cooper, M Wallace; J Wallace, Leonard, Molumby, Mahoney; Bradshaw; Smith
Leeds possible starting lineup:
Casilla; Ayling, White, Berardi, Alioski; Phillips; Costa, Dallas, Klich, Harrison; Bamford
We say: Millwall 0-1 Leeds
Millwall are always a tough proposition at home, but the loss of their manager and Leeds' quality and dominance in games this season could combine to hand Bielsa's side a narrow victory.
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