My default position is to basically not trust any rumor that comes out of Spain's hilariously partisan football media, but El Confidencial were one of the first, if not the first, to report on the Diego Costa transfer last year, so perhaps it's worth a minute of our time to see what they're saying about Antoine Griezmann.
"...the key move to give credibility to [Griezmann's] possible departure this summer has been the entry into action of Jorge Mendes, the 'super agent' Portuguese who manages the European market and maintains excellent relations with both the CEO of Atletico, Miguel Ángel Gil Marin, [and] Chelsea coach Jose Mourinho. Unlike other representatives, Mendes never stitch without thread..."
-source: El Confidencial via Google Translate
So never stitch without thread ("Mendes nunca da puntada sin hilo") basically means that Mendes means business. He doesn't do things just for the fun of it, the implication being that if he's inserted himself into the Griezmann negotiations, then there must be a real chance of something happening.
And that something is supposedly a transfer to Chelsea. El Confidencial claim that the (confidential) buyout clause is (a confidential) €65m, which is a bit higher (£46.5m) than the previously floated £43m. Then again, Costa's originally quoted price was around €60m as well and Mendes helped us negotiate that down to something bit more reasonable (£32m, which was less than €40m at last year's rates).
So, put this one under the "may actually be happening" column. Hair stylists in the SW6 rejoice!