Celtic remain dominant in Scotland, sitting eight points clear at the top of the Scottish Premiership after Sunday's 3-1 win at St Johnstone.

But it's a different story in Europe, with Rodgers' side crashing out of the Champions League in midweek thanks to a 2-0 defeat at Lazio - their 15th consecutive group match without a win.

• Stream the Scottish Premiership and more with NOW | Get Sky Sports
• St Johnstone 1-3 Celtic - Report
• Live Scottish Premiership table | Results | Fixtures

Rodgers said after that result that he is "very confident" of adding to his squad in January, adding: "We don't need quantity of players, you need quality."

Speaking to Sky Sports before Celtic's win at St Johnstone, Sutton said: "This season's Champions League campaign has been really feeble and disappointing. Celtic weren't prepared going into it.

"Celtic had a gentle group. Against Lazio the other night, they really lacked physicality and belief. The worry is whether that starts to filter into domestic performances.

"Brendan's come out this week and said the club needs to add more quality in January. We'll see what happens.

"I don’t think it's that easy to just go and buy quality, experienced players. Clubs down south are spending £30m, £40m on bang average players. Celtic can't afford that.

"Rodgers is basically saying, ‘this squad isn’t good enough’. He’s saying they weren’t ready and now they need to build by bringing players in."

Boyd: Why would Celtic board spend?

However, Kris Boyd questioned whether Celtic's board will sanction spending on older, experienced players in January when they remain the pre-eminent side in Scotland.

The champions have focused on buying younger, cheaper players in recent times, with Boyd pointing to the successful signings of Moussa Dembele, Odsonne Edouard and Virgil van Dijk.

But Celtic's summer signings have made a combined nine starts in five Champions League games this season, while the £40.7m profit the club made last season has led to calls from supporters for the board to step up their investment in Rodgers' squad.

Speaking to Sky Sports, Boyd said: "It’s easy to talk about January but Celtic aren’t going to have European football.

"Everyone tells me Celtic are the best team in Scotland by a country mile so, if you're the board, why would you want to go and spend a fortune in January to win the league again?

"Celtic have got enough to do what they need to do in Scotland and then they'll just bank the rest of the money. If you look at Celtic's balance sheet, it's worked.

"But I get why the fans are not happy. They want to go and watch their team perform in Europe but they're miles off it. Even under Ange Postecoglou, it looked attractive but they got battered."

But Sutton disagreed with Boyd's belief that Celtic's board would be reluctant to spend next month, saying: "The point Kris makes about Celtic not needing to sign players in January was utterly ridiculous.

"Of course Celtic are going to sign players if Brendan thinks they’re not at the level. They need to build again. That’s an admission that they got things wrong in the summer."

Rodgers: We need to add quality

Speaking after the 2-0 defeat in Rome which left Celtic five points off third-placed Feyenoord with just one Champions League game to play, Rodgers said: "There's no doubt that the squad will come down in numbers and obviously we need to add quality.

"There is no dressing it up, the group lost real quality players in the summer. That's something that over the coming windows we will look to improve on.

"The supporters have every right to demand the highest level. That's what we demand here also.

"The club have been so well run for so many years and, from a domestic perspective, that's been highlighted in what the club have achieved.

"I think the next step is the European side and that's why I came back, to hopefully be able to fulfil that in my time here.

"I felt when I came in here in the summer, with the squad that was already in place from last season, that we probably needed four players. By that, I meant four quality players to add to the squad.

"Now, how the game works, you lose players and there were players that were earmarked to come here prior to me coming here.

"That's absolutely fine, there's no problem, we have got young talents here that will develop and grow and that's the model of the club, but of course to continue to improve and grow you need to also have players who are established because they help bring those players up.

"I am very, very confident that we can do that over the coming windows.

"There's no doubt we have to add extra quality. We don't need quantity of players, you need quality."