Mr Johnson’s 80-seat Commons majority allowed his Brexit deal to sail through Parliament and become law ahead of Friday’s departure from the EU.
Mr Tusk, an outspoken critic of Brexit, told the BBC’s Andrew Marr Show on Sunday: “I was a little bit disappointed [at the result] because… the statistics are quite clear, it’s 50/50 when it comes to Remainers and Brexiteers in the UK, more or less.
“The Remainers [had] no clear leadership, they were not organised enough.”
Mr Tusk also hinted Scotland would be welcome to join the bloc if it ever becomes an independent country. Its ruling SNP is aggressively campaigning for a second independence referendum, something which Mr Johnson has so far refused to consider.
Mr Tusk said: “I want to stop myself from saying something too blunt. Sometimes I feel I am Scottish [in my heart]. I am very Scottish now, especially after Brexit.
“At the same time I have to respect sovereignty, integrity of the debate in the United Kingdom. It’s not my role to intervene, despite my sympathy.
“If something like the independence of Scotland happens then… it’s not automatic that [Scotland joins the EU].”
But he said the EU’s “emotional” response to the prospect would be “only empathy”.
In the same interview, Mr Tusk complained about the “hostile narrative against the EU” during the divisive Brexit debate.
He said: “The only two roles the EU played in the British narrative was bogeyman or whipping boy [as if] we were responsible for every failure. This is the main problem.”
Mr Tusk, who last year said there was a “special place in hell” for Brexiteers who proposed leaving the bloc without a plan on how to make it work, also praised the institution as a “miracle”.
After Brexit day on Friday, the nation has entered a “transition period” in which the UK government has until December 31 to thrash out its future relationship with the EU.
Read more from Yahoo News UK
Critics have insisted this doesn’t leave enough time to negotiate important points such as a trade deal, but Mr Tusk insisted “one year is enough”.
He added “we have to demonstrate goodwill on both sides” and that “we are good friends with Boris Johnson”.