It is not the EU President's gift to guarantee these changes.
All changes have to be agreed in a vote of the 27 members (it's a democracy) Donald Tusk can no more deliver the required result than John Bercow can tell make Parliament vote how he wants.
The problem here is of Cameron's own making. If the referendum is this year, there is not enough time for the concessions to be voted on in the EU parliament to make them binding.
The President is warning that if the referendum takes place before the EU Parliament has time to vote, then the vote might not ratify Cameron's changes.
Dave's dithering is the prblem. He should name a date, and give everyone a deadline they can work to (to get the promises properly ratified).
As a side note, you talk of so many EU promises renaded (I assume you mean reneged) on. What promises were they, and who made them?