Because he was signed after the transfer window was closed, and they can only extend it for a month.And maximum is 3 months. It is not Chelsea's fault this time.
5 assists and 4 goals this season, 14 assists and 7 goals last season and similar stats at Borussia M'gladbach. I think he is already proving, and he is only 21. Could he be better than Modric? I think so.