I don't claim to know Sundin's mind, but at the first round of the NHL Draft Fletcher indicated that the Canadiens have been granted "exclusive" negotiating rights until July 1. This suggests at the very least, some sort of gentleman's agreement between Gainey and Fletcher. Sure, Fletch may simply want a few extra days notice if Mats is going to leave....in such a case, he might be tempted to focus on a free-agent to replace some of the lost offense.
The other possibilities may include a similar permission for the Leafs to talk early with one of the Hab's soon to be UFAs...likely Streit and Ryder. Both players are young enough to fit into a 3-5 year plan, and both have been productive.
Another option might be an agreement to ship some future consideration player back to Toronto to offset a salary hit from giving Sundin a worthy paycheck.
The rumor for a trade deadline rental was Chris Higgins plus a pick....Perhaps Higgins is still there..but more likely is one of 3 or 4 older prospects like Ben Maxwell who are getting older and could be bumped further down the depth chart anyway as they feel the squeeze from below (newer picks) and additional NHL vets like Sundin.