TOKYO—A shortage of imported American spuds forced McDonald’s to institute limits on french fry orders at its 3,100 Japanese outlets today—removing medium and large fry orders from its menu, leaving small fries as the only size available.

But if a rationing system raises the specter of hordes of french fry fanatics clamoring for a fried potato fix, the scene was calm at the crowded Ginza McDonald’s in central Tokyo, where workers on their lunch breaks mostly took the restrictions in stride.

“I usually buy a Big Mac with medium fries so I’m disappointed,” said Kaoru Yamano, 24, who works at a trading company. “I probably won’t return until fries are back on the menu.”

McDonald’s also cut the price of its set menus to reflect the inclusion of fewer fries: a Big Mac meal that normally costs 669 yen ($5.71) now costs 619 yen ($5.29). It remains to be seen how the rationing will affect overall sales, but the chain has already had a tough year in Japan due to a food safety scandal. McDonald’s Japan subsidiary, 49% percent owned by US-based McDonald’s Corp, said in October that it expects to post its first annual net loss in 11 years—and that was before the potato shortage hit.

In sharp contrast to the french fry mania that swept Japanese McDonald’s in 2012, when teenagers bought hundreds of orders of fries and ate them off the table in an orgy of potato consumption, customers in the new rationing era seemed pretty blasé.

“I don’t care,” admitted office worker Kaori Ito, 28, who visits the chain a few times a month. “I usually get medium fries but a small portion is just right. … I’ll still come back.”

An employee at the fast-food outlet, who declined to give his name citing company policy, said despite the french fry rationing, it was “business as usual” for McDonald’s workers. “But we don’t know when the medium fries will be back. That depends on the US situation,” he said.

The shortage, which is also affecting the Japanese chain Skylark, was caused by prolonged labor unrest at US West Coast ports. Disagreements between the dockworkers’ union and international shipping lines have prompted slower shipments for months, forcing McDonald’s and other chains to resort to airlifting in frozen potatoes to try to ease the crunch. McDonald’s, even more than its fast-food competitors, relies on a largely outsourced global supply chain, which in good times contributes to higher profit margins.

“Potato S,” colloquial Japanese for “small fries,” spent much of the previous day at the top of Twitter’s trend list for Tokyo. Many users tweeted out photos of their meals before the Great Japan French Fry Shortage took hold. “There will be nothing but small fries from tomorrow, so I bought five large portions,” wrote Twitter user Kaito. “That’s a direct line to high blood pressure,” responded a user named chigatatu.