I'd say generally, the club did quite well with transfers under him. Sels, Lazaar, Atsu and Muto are the only ones I can think of as particularly bad value in hindsight.
For reference - the players signed under Rafas tenure (bold still at the club):
Ritchie (10m)
Gayle (10m)
Sels (5m)
Hanley (5m)
Clark (5m)
Yedlin (5m)
Darryl Murphy (1m)
Diame (5m)
Hayden (4m)
Lazaar (3m)
Games (£free)
Jacob Murphy (12m)
Lejeune (9m)
Joselu (5m)
Atsu (6m)
Manquillo (5m)
Merino (10m)
Dubravka (7m)
Slimani (loan)
Kenedy (loan)
Muto (10m)
Rondon (loan)
Almiron (20m)
Ki Sung-Yeung (£free)
Barecca (loan)
Fernández (6m)
Schar (3m)