@CroJack ... I agree with you that it is impossible to predict what would have happened with respect to Cullen's game time. This is why I used the qualifier "likely". I was making a projection based on 23% or so of the season. All other things being equal it's a reasonable projection to illustrate difference. A 1 or 2 goal difference over 8 games does not seem critical, but scale over a season and difference becomes recognizable.
The Cullen v Piroe debate is on the table because Martin has chosen to utilize the players in an either or situation up until 7 games ago, and now has chosen to utilize Cullen/Whitaker in an either or situation in the last two games, with Piroe being the constant.
@ivoralljack raised the question of what are the differentiating factors - and I also agree that frankly take away the goals and Piroe's offering is slim IMO. If we're discussing / debating what would be optimal then I agree with your assessment. However, Martin's choices are setting up the debate and Martin's choice has been Piroe is constant and others are options around him. The arguments are countering Martin's "logic" and choices. Piroe is not a franchise player I would build a team around, and we certainly have not attempted to do that, primarily I would argue because his strengths are not crystal clear.
An equally viable alternative would be: Cullen, with Cooper, Whitaker and Cundle/Fulton behind ... this following the logic that Piroe doesn't have the attributes to play wide-ish, not that Cundle really does either but at least he's more mobile and energetic, and Fulton is all around better in that role.
@CroJack is absolutely correct on the primary reason we lose games. We come under the cosh defensively because too often we have players becoming anonymous on the field so playing short handed or making unforced errors and one way or another we engineer ourselves into compromising situations.
United's first goal came about because Manning failed in a rash challenge in their half that allowed them to break quickly down that flank. Manning plays the ball into touch or contains instead of committing and all is well. United's third goal: had Naughton not bottled a challenge on the edge of the box that allowed a player to play through but instead put at least a toe into the ball all is well - we don't get to the point of Fisher's faux pas. United's second ... they finished well but nobody was ready for the second ball, nobody is thinking.
Martin is not ruthless enough when it comes to players becoming anonymous.