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.
Benitez is concerned about the depth of his squad and remains desperate to sign a central striker - ideally West Brom's Salomón Rondón - as well as a No 10, a centre-half and a left-back, but asked if there was money to spend or any deals were close he said: "I have no idea".
Benitez then said of Newcastle's fans: "They have to be concerned".
That fact has caused fury among the Geordie faithful for many years, and it appears boss Benitez is starting to align his frustrations with the fans.
Newcastle signed Japan forward Yoshinori Muto from Mainz on a four-year contract on Thursday, hours after manager Rafa Benitez hinted at concerns over a lack of transfer spending following a 4-0 friendly defeat.
The Venezuela worldwide had had a release clause of £16 million following the Baggies relegation - but the turn of the month has allowed that clause to expire, with Darren Moore now wanting in excess of £25 million for his star striker.
The Chronicle understand that Newcastle striker Dwight Gayle is part of the conversation between the two clubs.
Information from The Associated Press and Reuters was used in this report. "We were following him for quite a while", Benitez said.