Toews is not untouchable, period. He has yet to play an NHL game and people are calling him a franchise player. Ridiculous.
Its quite possible hes going to be an all-star for years to come, but i'll pass judgement when I see him play. Until then, he is worth no more than any proven player in the league.
Jamal Mayers is a proven player in the league, how about a straight up swap then?
Sure all Jamal Mayers has proven is that he is a legit 3rd-4th liner, but he has proven it, just like you asked for.
In all seriousness, it is pretty ridiculous offer, equivalent to some of the worst these boards have ever offered up. It serves only to illustrate my point, he is worth more than proven talent, because of his potential. Unless of course you mean he is worth no more than proven 1st line talent, which would still be debatable, but much more likely to be fair.