He also said the best way for the prime minister to demonstrate UK credibility on defence would be to use a meeting with Mr Trump in Washington this week to set out a plan for an "Anglo-French" force to help provide security for Ukraine.
Mr Johnson said he hoped Sir Keir would use his trip to Washington to "set out an Anglo-French deterrent force that can begin to provide the steel-quilled porcupine of future Ukrainian security".
Speaking to Sky News on a trip to Kyiv, Mr Johnson also said Sir Keir Starmer should hike UK defence spending up to 3% of national income "very quickly" - at least by 2030 "if not sooner".
Mr Johnson pointed to a deal that Washington wants to sign with Kyiv that will give the US access to Ukraine's vast mineral wealth - though the Ukrainians want US security guarantees in return.
Sir Keir is expected to set a timeframe as early as this week for when Britain will lift its defence budget to 2.5% of GDP from 2.3% - but Mr Johnson signalled that Mr Trump would be expecting a "bigger number".
