I do remember when the two of them were combining extremely well and were ripping it up for a couple of months. Things looked very good at that point. That was Martinez's first season.
Then Del got injured and never really staked a claim for being included long term in the first team under both Martinez and Koeman even though he had many chances. Of course Rom kept scoring and now looks even better at Utd. Perhaps they both needed better players around them?
As for Niasse, what has he done week in, week out since he's been introduced in the fold? I remember he came on and got a brace against Bournemouth and then started the next game and looked shit.
I hope he does come good, but I think his best role is that of super sub. Just like at Hull when he got 4 in 17 games.