Brennan is a fine player with bags of potential - lightning quick (provably - that stat that showed he had the second fastest top speed in the division), quality finisher (almost always hits the target, even when running at pace) and getting better at finding space.
However, he didn't make the team for the Chelsea or Arsenal games and his lack of physical strength (especially his reluctance/refusal to challenge for headers) means you need another striker alongside him. Awoniyi has looked poor on the left, when we've played split strikers with MGW behind and MGW is wasted on the left when we've played more of a front three. I can see why, if you are taking a hard-headed decision, Johnson would have a price for the club and why they might not want to push his pay up to the levels of some of the top earners if he's not a must pick.
Hopefully, he and Cooper can find a way to make it work as it seems like a massive shame if he does leave.