The 27-year old striker has largely unfulfilled his potential throughout his career but McClaren is confident he can bring the best out of him at iPro Stadium during his season-long loan from Championship rivals Blackburn Rovers.
"Leon has not developed his full talent, although he has had injuries," the former England boss told the Daily Telegraph.
"He seemed to find his home at Newcastle United, where he scored goals and looked a good player.
"We want Derby to be his second home where he feels comfortable and starts fulfilling his potential again.
"He is capable of doing that. He is only 27 and we believe in him."
The seven-time Republic of Ireland international enjoyed his best run of form during his time with the Magpies when scoring 10 goals in 46 appearances but has struggled for consistency at his other six clubs which include Southampton, QPR, Sheffield Wednesday, Bournemouth, Yeovil Town and Blackburn.
Best had his longest spell with Coventry City between 2007 and 2010 where he managed 23 goals in just over 100 outings.
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