The Lions boss was particularly unimpressed by his team's display in the final 20 minutes of the contest, but was quick to highlight the quality of the opposition.
"It was disappointing to concede the goals we did. I thought we lost our way in the last 20 minutes but there is no getting away from what a good team Tottenham are," BBC Sport quotes Harris as saying.
"We are disappointed because we have been beaten 6-0 at Spurs. This is the quality you are playing against. If we can use this experience then the standards are there that we need to set, individually and collectively."
The quarter-final defeat was Millwall's first in 19 matches in all competitions.