I don’t know what the other poster’s definition of ‘the grade’ is, but from my perspective it’s contributing regularly and being important to a successful, trophy winning side, like KDB, Fern, Walker, Vinnie, Gundo, Silva, Mahrez, etc have.
So for me I’m not sure whether Doku, Savinho, Nunes or Lewis will ever be at the level where they are indispensable and key to a trophy winning side.
In the cases of Gundo, Silva, Mahrez etc, they never started off as being indispensable, they worked toward that status, plus they were older when they arrived at City than Doku, Savinho & Lewis, so I don't see that you're comparing apples with apples.
In terms of Mahrez, at the same age he was playing lower league football in France, & throughout his City career, many on this forum referred to him as FFS! Riyad still remains the most underrated City player of recent times imo, & his true value was only felt after he left.
Again, being "indispensable" is subjective. Did many class Gareth Barry as indispensable? Not to my recollection, but like Mahrez, when looking back he definitely was, again imo.
Many will also associate being indispensable with being a superstar. The two are totally different things. Many players who're viewed as indispensable often only reach that status after they leave.
However, whilst they were here, they were the glue that gelled the team together, which allowed the likes of KDB etc, to flourish & shine. Most of that unsung work goes unnoticed, until it's missed.
Back to Doku, Savinho & Lewis etc. They're at the beginning of their City journeys & I feel it only fair that like for like comparisons are made. Mahrez was 27, & had already been crowned PFA player of the year by the time he arrived at City.
A better comparison would be; at what stage was Mahrez in his career when he was 20, 21 & 23? We need a sense of perspective. It's not the fault of Doku, Savinho & Lewis that they have to be heavily relied upon because of our frankly poor succession planning.
Instead of blaming our young players, I'd say you'd be better off looking toward whoever it was who allowed our injury-prone squad to age, without starting our rebuild when it was plainly obvious it was needed.