If we were to sign a fifth forward to cover/compete with Ekitike (assuming Diaz leaves with a direct replacement inbound for the LW spot, e.g. Rodrygo), I don’t see that player being a pure No9 like Mateta. Think it would be a versatile forward who can get minutes across the front line.
I still have a feeling we’ll end up with four senior forwards (Ekitike, Salah, Gakpo, versatile Diaz replacement), with a combination of Frimpong, attacking midfielders and maybe Ngumoha filling in as needed. But if we can stretch it to five, I think Nkunku could be a good option, so long as he’s someone Slot actually wants/rates (i.e. not Chiesa).
On the injuries, he’s only had one season seriously affected by injuries, his first at Chelsea. Otherwise he’s been pretty reliable. He was excellent at Leipzig, is versatile, and apparently available for a decent price.