"We need more players," Moyes said, according to Sky Sports. "Our squad's not big enough and we need to get more quality in some areas where we can improve.
"I'm not sure it's going to happen, but it's certainly not because we're not trying."
The comments came despite a comfortable night at Deepdale, where Thierno Barry scored a hat-trick as Everton eased into the third round, where they will face Wolves. Moyes has already brought in a number of new faces this summer, including Christian Nørgaard, Brennan Johnson, Merlin Röhl, Tyrique George and Hayden Hackney, several of whom have already featured prominently in the club's opening weeks.
Nørgaard in particular is expected to help fill the gap left by Idrissa Gana Gueye in midfield.
Despite that recruitment, Moyes's frustration reflects a summer in which Everton have moved more slowly than he had hoped. Earlier in the window, the Scot had said he wanted as many as five or six further additions after completing only a handful of deals, describing it as one of the busiest and most demanding transfer windows of his managerial career.
He has also lost key squad players this summer, including centre-back Jarrad Branthwaite's understudy Jake O'Brien, who is set to leave, adding further urgency to the search for reinforcements.
With the deadline fast approaching, Everton face a familiar late scramble to turn Moyes's wish list into fresh faces before the market closes, with a new right-back still understood to be among the club's clearest priorities after years of searching for a long-term solution in that position.