due to their ffp punishment from 2 years ago, they cannot have a full squad in the cl. they can only enter 22 players, 2 has to be youngsters (u21 i guess), and regarding the last 2 year's transfers, they also have some limitations. for instance, that's why they cannot buy vrsaljko outright, because his full 25m transfer fee would brake the balance of the last 2 transfer seasons, while a ~8m loan wouldn't. and that's why they would loan vidal for his 1st year, too.