On hattrick you have those situations too (they have 1 mil users).
When you buy a player your team spirit (ent) will (can) drop, the player don't become any penalties on his GS.
I thought it would be the same on BB, but seems they have an another idea, we will see how long the penalties on GS will be, and how strong.
Yeah, HT went the different direction and decided to give a bonus to players who are on the team a longer time, rather than a negative to players who are on the team a short time. For TL moves, the team spirit drop can be avoided if you happen to narrow your search to players with the correct personality traits. But the comparison isn't a real apples-to-apples one because HT has already attacked the monster player angle in a different way, by changing the training so that training slows down as skills reach high levels, and the skill drops speed up on higher level players (and are age-independent). The other significant difference is that there's not nearly as much of a postseason in HT as in BB; in BB, there are playoffs, relegation series as well as the Cup and B3, but in HT, there are far fewer teams who would have incentive to rent-a-team. It could happen for teams in late rounds of the Cup, or perhaps the Masters, and then the teams in the promotion/relegation matches, but because only half of the league champions need to play those and only the fifth and sixth place teams play relegation matches, the problem isn't nearly so severe.