Jackett, 51, told reporters only a minority of fans were guilty of such offences, although he did admit even that is "too much".
The club vowed on Monday to reopen an investigation into allegations of racial abuse aimed at Diouf, with Sky Sports News airing damning footage as part of an investigation into racism which identified 56 examples at Millwall.
As a result of that footage, Gerrard Scanlon, 53, from Enfield, north London, was charged with a racially aggravated public order offence on Tuesday.
Jackett said: "We weren't surprised (by the documentary).
"The chief executive had had information that the programme was going out. Was I surprised at the extremity of the content? Perhaps.
"A minority is still too much but it does reflect society unfortunately."
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