If we had a really good passing 8 behind him I think we might get away with playing him in the 10 in a similar role to how Dele was deployed for us, especially if we were flanking him with 2 more creative progressive dribblers like Savinho and Kudus, however we definitely would need a real upgrade at 8 to be able to carry him in that way IMO, I think he has been played wide for us because his ability to contribute in the final third is unquestionable but his inability to effectively contribute to the build-up phase of play sees him shunted out there as damage limitation, we have the incredibly hard-working Porro on that side who has been exposed himself trying to cover for Brennan's deficiencies in that department from time to time.At Forest he often played upfront in a front 2. They built their attack around him and always had him coming centrally.
Under Ange, we always played him wide. His goalscoring ability is elite, the numbers speak for themselves! But he's bang average as a winger, in fact below average. His 1v1 dribbling is poor and he has very poor numbers for successful take ons. He also has a poor crossing ability where going low and hard is his only real option. Basically he is very very easy to nulify on the wing for an opposing fullback. They want to see Brennan out wide as he's little threat. And don't forget that we played with inverted full backs for the last 2 seasons so Brennan has always had to stay wide and hold the width for 90% of games which never suited his style.
However, if Frank plans to use him more centrally, either running behind Solanke or else in an inverted wing role (think Mbeumo at Brentford who rarely hogged the touchline) then we could have a really really effective player on our hands who can score even more goals as won't be stuck on the touchline trying to play as a winger during the game where he is totally ineffective.
So I disagree that he's gotten worse then he was at Forest. I think he's always been a very poor touchline winger and from now on he should be used much more centrally. I think Frank will get the best out of him. He's one player I'm not concerned about at all. I think he's the most underrated player on this forum.
I think 2 up front can only ever be a situational tactic in this day and age, being outnumbered in midfield against highly system-oriented sides often just leaves your attack stranded and unable to contribute aside from on the counter.

