The Lions' record goalscorer had been sidelined since mid-November with a back injury but made a surprise appearance as a substitute during last weekend's 1-1 draw with Southampton.
With a full week of training behind him Harris could now partner Steve Morison in attack at Boundary Park.
Millwall are still without fellow frontman Gary Alexander, who was this week ruled out for three months after a heel operation.
Midfielder Marc Laird faces a late check on a calf injury but defender Andy Frampton is not expected to recover from his calf strain in time.
Ipswich loanee Liam Trotter keeps his place in midfield after a debut goal against Saints.