Interesting to see so many bring up Firmino in this discussion. Despite only scoring 1 goal at home this season, he demonstrates a much better center forward play than Martial, which is an essential element in modern days. Rather than just goalscoring, you'll want your strikers to be heavily involved in the link up play, hold off opponents and protect the ball up front, drag opponents out and create spaces for teammates etc.
For me Martial just hasn't done enough in these aspects. I don't think Ole is satisfied either from the way he compliments Ighalo, and suggests he offers something we've been lacking since the start of the season. When other top strikers are in bad form, they can still create chances and spaces for their teammates; but when Martial is having a bad day, it's like we are playing with a man down, and that's one of the reasons why we struggle to break down smaller teams.
Also, if you really want to play the Liverpool way, you'll need great attacking fullbacks to take up the spaces left behind by the wide forwards cutting inside. I don't think there's much debate that Wan-Bissaka, Shaw and Williams aren't capable of posing offensive threats like Arnold and Robertson do. In fact, I think we're more suited to the City way where wingers take up the wide positions allowing the midfielders to go forward, unleashing the attacking quality of Pogba and Bruno.
Having said that, I don't think it's urgent for us to sign a striker especially with Ighalo as a reliable backup, and Martial is going to keep his spot at least for the next season. He'll have all the time he needs to improve, but we have to reconsider our choices in the long run if little progress is seen.