At a meeting of the UEFA executive committee in Zagreb, the body warned co-hosts Poland and Ukraine that work on new stadiums and infrastructure must be increased.
Tusk said: "I heard words that confirmed that the challenge is serious, but also give reasons for optimism. Nothing is lost."
Poland is expected to reveal plans for a new national stadium in Warsaw today. The government plans to present a new preparation calendar next week.
The choice of Poland and Ukraine as the first east European countries to host the European championship sparked criticisms in some quarters concerning the hosts' poor infrastructure, which they argued could not be updated in time for the event.
Recent political changes in both countries have also exacerbated problems, shifting attention away from the preparations.
