After Chelsea recalled Tomas Kalas from his disastrous loan spell in Germany last season, they sent him to Middlesbrough in search of playing time for the remainder of the season. He made 17 appearances there, and seemingly impressed manager Aitor Karanka enough that the club is finalizing a season-long loan deal, which would see him return next season.
Even before Karanka took over at Middlesbrough, the Championship side enjoyed a good working relationship with Chelsea. The two clubs have worked out loan agreements involving Chelsea youngsters in each of the past three seasons, including four different players spending time at Boro during the 2014/15 campaign.
Kalas likely wouldn't be alone at Cobham north, as Chelsea teammate Nathan Ake has also been linked to a season-long spell with Karanka. It seems quite likely that the Blues will send the maximum allowable compliment of three prospects to Middlesbrough for a second straight season, though it's a bit unclear who else would be a logical fit, aside from Patrick Bamford, who is highly unlikely to return.