That's because Ighalo is more a predatory striker type player than Martial.
Martial isnt the hardest of workers - but that doesnt make him straight up lazy & the fact that he wants to be involved in the build up of play is the sentiment of that. He wants to play deep and be a part of the build up process - a cog of the machine rather than the spearhead of the attack. He is a support striker and a false 9 aiming to get the wider forwards like Rashford & Greenwood to score more.
When people saw Ighalo playing upfront for us - Rashford was injured, Greenwood was still being inconsistent and people seemed so surprised that Ighalo looked like this wonderful striker that Martial isnt - it's because we shifted to a team where our CF (martial) had to lead the line when Rashford got injured rather than playing a supportive one to get Rashford, Greenwood and James to be Goalscorers.
The fact is Lukaku would be just as good as Ighalo upfront when playing against poor teams & asking him to lead the line as a predatory striker - but how would he play when he is asked to interlink with the uninjured Rashford and Greenwood either side of him in a 433 or a 4231? Decent in some games maybe but sure there will be just as many complaints about Martial when playing next to Rashford & Greenwood as there would be Ighalo or Lukaku playing in that formation.
We hardly have fullbacks that can cross, we hardly had a midfielder for the whole of the season that was fit enough to provide an assist, we had our first good CAM bought after god knows how long, our LW and RW are actually LF and RF playing forward roles striking from angles to become more efficient like aubameyang does for arsenal, we have arguably one good ball playing defender, our goal keeper isnt the best at long passes either - dont underestimate what a creative striker or a supportive striker can do when we have first team decisions like this and our goal scorers are playing out wide.