If we win, we all win together, but if we lose, I have to turn my focus on my team to see who was of the greatest value, who was of the least value. If we win, we all win together, but if we lose, I have to turn my focus on my team to see who was of the greatest value(during that game), who was of the least value(during that game). It's past tense, discussing the match/game they played. So the players value wouldn't be the same all the time, after different games there would be a different player really shining, or playing better than others. Your version with "has" suggests the "value" of the player doesn't change and that its a set amount. if you read it with the brackets, it will make more sense. This is what he is really saying, but he is missing that part out. Who was the most valuable to us that game? Who was the least valuable to use during that game?

He is talking about what happened when the team lost. Who was most valuable in that game, who was least valuable in that game. "Who has the most/least value" would refer to who has the most value at this time and going forward.