Thanks for proving my point by bringing Mitoma, March and Enciso into the conversation... none of whom fulfil the number 9 role which De Zerbi's 4-2-3-1 relies so heavily on. It's comparative to grouping Lamptey with Dunk and Webster because they all play in defence.
Welbeck and Ferguson are...
De Zerbi used Trossard mainly as a centre forward, so there was room for both him and Mitoma - as we saw from the Chelsea game onwards when Mitoma made his first league start.
The point about Trossard is that by selling him, we were left with only an 18 year old who needs to have his game time...