"I don't think that net contributors are ready to take on all of Britain's contribution," Guenther Oettinger told business newspaper Handelsblatt.
"At the same time, Poland and the other net beneficiaries will not accept it if help for structurally weak regions were to be cut by nine billion euros annually.
"A compromise will have to be found," he added, AFP reported.
With new financial burdens arising from the refugee crisis, embryonic collective defense plans, and the fight against terrorism, the EU could slash traditional areas of spending, such as agriculture subsidies.
Oettinger told Handelsblatt that he was opposed to a specific tax to pay for the EU, as suggested recently by former Italian prime minister Mario Monti.
Instead, part of the bloc's cash could be tapped from fuel taxes in the member states -- "one or two cents a liter," he suggested.
London will continue to finance EU projects it had a hand in creating even after its planned final departure date in 2019, Oettinger said.
"British payments into the EU budget won't end suddenly when they leave," the recently-installed budget chief said.
Brussels is gearing up now to decide on spending in its next seven-year budget, set to come into effect in 2021.
But Oettinger suggested that Brexit could be an opportunity to shorten the planning period to five years -- a longstanding demand of European Parliament lawmakers that would see budgets match the length of their terms in office.
The EU could bring forward the start of its next budget period to 2020, the German commissioner said -- leaving Britain on the hook for EU contributions until the start of a new budget in 2026.