Muto, who has scored 20 Bundesliga goals for Mainz over the past three seasons, is Newcastle's fifth signing of the summer, with Martin Dubravka, Ki Sung-yueng and Fabian Schar having also joined on permanent deals, and Chelsea's Kenedy starting a second loan spell with the club. "I don't think we'll lose any players at the moment", Benitez said.
And after Newcastle's 4-0 friendly defeat to Braga in midweek, Benitez delivered a blunt transfer update which left many supporters anxious over his long-term future at the club.
Muto completed a medical and agreed personal terms last week, but his move was only completed yesterday after receiving a work permit.
"I am very happy to be a player for Newcastle United", the 26-year-old said.
Earlier this week, Japanese global forward Yoshinori Muto joined the Magpies from Mainz in a deal worth £9.5m following the departure of Aleksandar Mitrovic to Fulham.More news: Trump would fight any USA special counsel subpoena in court: president's lawyer
In pre-season, the young defender has started against Bayern Munich, Arsenal and Atletico Madrid and not looked out of place, but that playing time has been largely down to the absence of PSG's World Cup players. We did not do things the way we have to.
"You're a very courageous man if you go out and criticise the owners, which to be fair is what he is doing", said Allardyce. We were poor and we made too many mistakes.
"Then we finished 10th and credit to our players, but I still thought we could do something more to be closer".
Most Newcastle supporters echo the concerns of the man who brought them back to the Premier League in his first full season in charge and kept the club in England's top flight the next year.