Anything done with the goal of affecting or managing a player's skill level is manipulation. The only thing you can do is not help them improve.
If you think your player is better than their number, and do things like not play them to keep them from demonstrating that ability, it is manipulation. If you play them with the hope that they will get destroyed by a far superior player, it is manipulation as you are trying to manipulate the data in the system. Sometimes you can't avoid a match up with a world-beater, and in those cases you are not manipulating, but if you're actually setting your player up to take a big loss, you're cheating. If you play them against a better player to see how they will do, it's not manipulation - you don't know what the outcome will be, and that information is useful to us, too.
Many LO's distinguish in their bylaws the difference between a forfeit and a concession. If you have a player available to play who doesn't put you in jeopardy of breaking the 23 rule, but you choose not to play them, it is a concession and could be a sportsmanship issue (depending on the reason, like not wanting to give up more than the 15 forfeit points to your opponent, or not wanting your player's skill level to change, or not wanting to affect their MVP standing). It also could result in a different score than the standard forfeit. You are depriving someone on the other team of the opportunity to play a match, so you better have a good reason.