The big players historically hold a lot of influence at Chelsea and while I would never doubt their attempts to win matches for the club, their levels undoubtedly change depending on how they feel about a manager at the time. They're either having him or they're not, and once they're not it spells borrowed time for the guy in charge.
The fear for English players has always been that you might risk a place in the national team if you go abroad but now with all the technology we have, and social media, you are able to watch goals and assists every week, which means his performances are just as noticeable as anyone's in the Premier League.
There are many clever contractual ways to let a young player continue his career elsewhere while still having first option to bring them back in future. Contracts can include a buyback clause such as Barcelona had with Cesc Fabregas, or a sell-on clause where the development club benefit financially from all the work invested in the young player.