That isn’t strictly necessary; while America was isolationist with regard to Europe, they were far more involved in Asia, maintaining the ‘Open Door’ policy with regard to China as well as being very concerned by the threat Japan’s expansion posed to The Philippines.
The Lytton Commission included an American; the commission was set up by the League of Nations to determine the facts as to what was actually happening in Manchuria and who, if anyone was to blame. The commission’s condemnation of Japan is what prompted their withdrawal from the League.
The United States had also co-sponsored the Kellogg-Briand Pact of 1928 which was specifically intended to outlaw wars of aggression. Among the nations that had signed the pact was Japan, so there was definitely a basis for sanctions in International Law without America being in the League.