An easy yes for me, Ighalo occupies the 2 central defenders like a proper 9, not on about him scoring but by occupying those defenders he free up space for other players, allows gaps form through balls and runs, in addition his hold up play and touches around the box have been good (generally better than Martial's, whose hold up play is non-existent).
Without that no. 9 doing a no. 9 job then nearly everything we do goes down the left, because this normally involves the talent Rashord and Martial there is nothing to stop one of the central defenders coming across to back up the RB, blocking up the left... it becomes very predictable and easy to defend against, hence our performances against lower table opposition, Ighalo may not be the best no. 9 in the world, but right now he is the only one we have, he loves the club and has a brilliant attitude.
The answer is based on which type of player does what we need, right now Oghalo is the only out and out striker we have near the 1st team, (I wouldn't be against signing him based on the evidence so far, providing the wage was reasonable and the fee was pretty low, we need bench players who are happy to be there).
Martial is a luxury player who does not contribute nearly enough to the team when he is not scoring.... TBH he is a bit of a wet lettuce at times, especially if things are not running for him.
If we had a team brimming with talent I would rotate Martial and Rashford down the left wing, but for me we are much better playing a proper striker up top.... kind of makes you wonder why Lukaku happened?