Not solely but there are some good examples this season where managers have come in and immediately found vastly improved form, which continued over a decent sample size. Such as Tonda, Clement and Matos.
Also Boro were on a downward slope under Carrick, sold couple of their best players but Edwards and now Hellberg getting so much more out of that team consistently. Lampard at Cov another example. Ipswich under McKenna. Quite a few examples.