At the top of your .htaccess file, before your existing WordPress directives, you could do something like the following to redirect the old permalink:
RewriteRule ^\d{4}/\d\d/([\w-]+\.html)$ /$1 [R=301,L]
The RewriteRule pattern matches against the URL-path less the slash prefix.
\d{4}/ – matches the 4-digit year, followed by a slash.
\d\d/ – matches the 2-digit month number, followed by a slash.
([\w-]+\.html) – matches the postname and .html extension. The surrounding parentheses make this into a capturing group which is then referenced in the substitution string with the $1 backreference. [\w-] matches characters in the range a-z, A-Z, 0-9, _ (underscore) and - (hyphen). If your postname can contain any other characters then these will need to be added to this character class (although the hyphen must appear last).
Test with a 302 (temporary) redirect to avoid caching issues in case anything goes wrong. 301 (permanent) redirects are persistently cached by the browser by default.