The midfielder arrived at the deadline on loan from Atletico Madrid but struggled against Villa in his introduction on Saturday
Thomas Tuchel said Saul Niguez made "big mistakes" in his Chelsea debut on Saturday against Aston Villa, but the manager took responsibility for the circumstances.
He picked his line-up thinking the midfielder would be able to adapt right away to the pace of play before replacing the Spain international at half-time of a 3-0 win.
Saul, on loan from Atletico Madrid, did not train in pre-season with the Blues as he arrived just before the transfer deadline.