Wow, good deal for both teams.
You can bet Yzerman exhausted every opportunity to dump additional cap space but could not do it without parting with too much of their young assets. You can also bet that Drouin wasn't popular with the club/teammates due to his actions over the past 2 years. That said, he is a potentially elite talent and will get tons of minutes and opportunity in Montreal. Tampa has some nice prospects coming in on the wings too. On value alone, right now, Montreal wins this trade.
Sergyachev is young, cost-controlled and should thrive in Tampa' deep defense core. He is a gamble right now, though but if he scores 10g-20a next year it will be considered a wash, value-wise.