I think you're looking at these guys with the wrong perspective. They can do well against teams for 20 mins, after the other team has tired but for years have proven to not be good enough to start for their respective teams. We don't need bench players. We need definite improvement of the starting 11.
I'd say we can turn over a maximum of 4-5 players without effectively starting again next year. (i.e. wasting all the improvement we may or may not have made this year depending on your view.)
Left back, 1 CM, 2 attackers. This would be an easy way of not disrupting the team dynamics but still improving across the park. Then, only if we find real quality, we replace Mullen or another midfielder.