Chelsea confirmed the signing of Namanja Matic moments ago, and also answered the question of what jersey number he'd be wearing. According to the announcement of his signing, Matic will be wearing #21.
That number was currently vacant, but one of Chelsea's players on loan was wearing that number last season. Those of you hoping for a Marko Marin return so that you can dust off your most recent #21 jersey will be disappointed, if the German ever dons a Chelsea kit again, he'll likely be doing so with a different number.
Forgetting Marin for a second though (and that's easy enough to do), the new #21 will be a welcome addition to a Chelsea midfield that's been in need of help fr a while. Here's hoping there are no issues transitioning back to the Premier League, and that his jersey becomes a big seller.