If he unilaterally pulls out of NAFTA, yes and it will probably be negative. If he is willing to renegotiate, then it drops to a perhaps and we might even be able to wrangle something positive out of it. So it all comes down to how he really handles NAFTA (versus the rhetoric he used in the campaign).
That said, it once again proves that we need to be diversifying our trade. CETA was supposed to help with that but it's stalled and, with the Brits going hard Brexit, we will have to do a separate deal with them now (and that's already being discussed, in fact, according to one article I read). China is another that Trudeau is pursuing but that is fraught with perils of its own. TPP might go on without the US, but the only major trading partner in that for us is Japan if the US is, indeed, out.