The deal does not include an option for Dortmund to make the deal permanent.

The Blues are likely to be open to selling the 21-year-old in the summer as they look to comply with Financial Fair Play rules after heavy investment in the past three transfer windows.

Chelsea accepted a £31.5m bid from Burnley for Maatsen last summer, but he did not want to make the move.

The Netherlands Under-21s defender has made just three starts this season, with a further 12 appearances from the bench.

During a loan stint at Burnley last season, Maatsen played an important role in the Clarets winning the Championship and was named in the league's team of the season.

Chelsea are open to signing a striker in this window but are unlikely to spend the money it would take to bring in Victor Osimhen from Napoli or Brentford's Ivan Toney.

Mauricio Pochettino's side take on Middlesbrough in their Carabao Cup semi-final first leg on Tuesday.