Murphy, 24, is Newcastle's third summer signing after winger Christian Atsu made his loan move from Chelsea permanent and centre-half Florian Lejeune joined from Eibar.
Neither club has disclosed the fee nor the length of contract for Murphy, 22, who enjoyed his breakthrough season at Norwich last season.
"He (McCarthy) is a good player but we now have a lot of them at this club", Benitez said.
Murphy saw Newcastle beat Dutch side PSV Eindhoven 2-1 at St. James Park, with goals from Alan Shearer and Gary Speed either side of a Mateja Kežman penalty.
Benitez added: "We hope he can show us the passion, commitment and ability that he showed in the Championship last season".
The Shape of Water Trailer: Guillermo del Toro's New Fairy Tale
So they're trying to keep me a secret from the Russians. "What she goes through...you're going to be rooting for her every step. In the hidden high-security government laboratory where she works, lonely Elisa is trapped in a life of silence and isolation.
Murphy is training in Ireland with his new team-mates.
United have also watched Murphy's twin brother Josh, a forward at Norwich.
"We are talking about different things in the Premier League and we have some clubs asking about him", Benitez told the Irish Examiner. You can't have players that think this is easy because they are the only one in that position.
"But, in this case, we wanted the type of player (Jacob is) and were happy with that".
The Magpies have made four signings since their return to the Premier League after winning the Championship.
- Former Crystal Palace boss, Sam Allardyce open to global job
- Second medal for Aurelie Muller
- " "Un chrétien israélien accusé de " crime d'honneur " contre sa fille
- Yum! Brands, Inc. (YUM) Shares Sold by Heritage Investors Management Corp
- China urges global support for proposed Seoul-Pyongyang talks
- New gameplay trailer released for The Evil Within 2
- Trump slams 'dishonest' media over 'sick' reports of Putin exchange
- Spanish banker found dead with gunshot wound
- Coca-Cola Company (NYSE:KO)
- Pas de décision avant septembre — Tiers payant généralisé