Eddie Howe concedes he “can’t guarantee” retaining all his key players in January and is well aware Newcastle are striving to remain on the right side of Premier League spending regulations.

Howe is no fan of the profitability and sustainability rules (PSR) that restrict the buying power of clubs such as Newcastle with still-developing commercial revenue streams. As the St James’ Park board struggle to play catch-up with rivals including Manchester United, Manchester City, Liverpool, Arsenal and Tottenham off the field, the purchasing might of their Saudi Arabian-majority owners is heavily restricted.

“The days when teams don’t sell big players are gone,” said Howe as he prepared his team for Saturday’s trip to Brentford. “I think our revenue streams aren’t where we want them to be. We’re still trying to grow them, so I think that trading in and out is going to be essential.

Related: Postecoglou responds to Spurs fans, Mourinho hits back at Guardiola: football news – live

“Hopefully it’s not going to be players that we’re forced to sell but you can’t guarantee anything. The issue with PSR and our position is that nothing is clearcut. Nothing is absolutely fixed; there will be lots of twists and turns in January. But we want to try and improve the squad.”

Howe avoided the imperative to sell one of his star names – the widely coveted Alexander Isak, Bruno Guimarães or Anthony Gordon – in the summer by reluctantly agreeing to sell two promising young players in Elliot Anderson and Yankuba Minteh to Nottingham Forest and Brighton respectively.

Right now, Howe hopes Isak, Guimarães, Gordon and the rest of his team can improve on their current 12th place and oscillating form by regularly reprising the quality that earned them a 3-3 draw with Liverpool on Wednesday. “The better players in this league are the better players because they repeatedly do it,” said Howe. “It’s easier to do it on the odd occasion and show how good you are in phases.

“Attitude is everything to any team. Against Liverpool you saw the correct attitude, the correct mental application. We need the mental quality to be as good as we are in terms of quality.”

Newcastle have often impressed against leading sides – they have beaten Arsenal and Tottenham at St James’ Park – but slipped up against hitherto struggling opponents. “This has been in our make-up for a while now and it’s a frustration because we know what we’re capable of,” said Howe. of the inconsistency. “The missing ingredient is that you deliver against whoever you’re playing.”