Nylander isn't getting exposed, he's been productive his entire career, even away from Matthews.
If anything, it's people who were underrating Kapanen and who kept pulling up his numbers despite playing grinding/defensive minutes, less than 10 minutes a night, on the 4th line with non-NHL'ers like Moore centering him. You could put your best player on your team out there, in those circumstances and let's see how they'll fare.
As for the OP's deal, it isn't terrible. I wouldn't trade Nylander though for anything realistic to Minnesota. Especially with Matthews gone, it opens up minutes for him at C, which is the ideal circumstance (aside from the injury of course) from a development standpoint. It was also Matthews injury last year where Marner stepped up and his game took another level- and he hasn't looked back.
I should say, I prefer the OP's deal to Dumba by far as well. I'm not a huge fan of Dumba though.