The short answer is yes, if "long term" means not going out now and looking for a replacement. Whether that means long term in the way of, say, Rooney's tenure is an unknown at this time.
I've always been worried about relying on him as our main number 9. His speed and especially his dazzling technical ability with the ball are undeniable, and that's what makes me hope that he can be our settled main striker. The bit that worries me is his inconsistency, which seems to be more moodiness than drop-off in form, something I think has been manifest since he first arrived under Louis, so can't just be attributed to Mourinho - at the time, I put it down to immaturity, but he's not a 19-year old now. In that respect he reminds me of Nani, who would sometimes be outstanding and at other times seemed bored (although with Nani I suspect it depended on who we were playing).
Of course, the answer is that kind of player needs to be well-managed, something SAF did supremely well with Nani and others. Martial has been hampered (and he's not the only player) by Mourinho's insidious "do-I-trust-you-or-not" management practices. Ole's much fresher and straightforward style, which derives a lot from SAF, has, in my view, already helped Martial to improve, as has the addition of better players, and especially Bruno.
The surprise factor has been Odion Ighalo - very few people would have predicted his signing or impact, regardless of why it came about. If it's possible to keep him (on sensible terms for all parties) at least for a season or two, and with Greenwood as a highly promising prospect, we will have a good spread of options without overusing Rashford again as we have done.
Assuming we continue to progress and Martial wants to stay, we can judge after probably two seasons whether he's is good enough to support the ambition to achieve success domestically and in Europe or whether we need to look elswehere.