A defender is definitely needed. Lindelof is getting better, which I can't seem to say the same for Jones and Smalling. Bailly is good, but too injury prone. Not even mad that we're willing to pay 80M for for Maguire. Van Dijk was sold to Liverpool for 75M and evidently it paid off, BIG time.
I think we're fine, in terms of attacking. If Lukaku leaves, it's really not the end of the world. Personally I prefer we play Martial up top anyway, and give him his no. 9 shirt back. People tend to forget how much of a monster he was up front when LVG first brought him in.
We need to stop stalling with Bruno Fernandes and just pay Sporting. He definitely seems to want to join.
So yes, I think a defender would massively help our odds of making it to top 3.