I'd normally suggest to use red packs at times I would consider important, where it is an important long term injury to one of your best players or you have 1 or 2 important games coming, If I'm playing 3 or 4 teams in the bottom half of the league and playing 1 or 2 CL group games when already or just about qualified for KO stages, I don't bother using reds then because it is just a waste. But thats my team though, mainly 6*/7* players in first XI. I'd still win my league if I didn't play my best 4 players for the rest of the season.