Watford have confirmed the sale of Matej Vydra to Derby County following a 3-1 loss at home to Arsenal in the Premier League on Saturday.
Czech international Vydra was a key member of the team that saw Watford earn promotion to the top flight in 2014-15.
But he spent last season on loan in the Championship with Reading and will now return to the second tier with Derby, signing a four-year contract for an undisclosed fee.
Santi Cazorla's penalty, together with goals from Alexis Sanchez and Mesut Ozil, saw the Gunners record a simple win, Roberto Pereyra netting a consolation for Watford in the second half.
"The second half we played good and should have scored more ... some episodes in the game did not go in our favour," Watford manager Walter Mazzarri said.
"[Petr] Cech played incredible for them, saved a goal two or three times but I am very happy with how our team played.
"A lot of players are new so we have to prepare later. The organisational things are still missing ... and this really didn't help."