Kieran Tierney is Celtic's 'only top-class defender', says Alan Stubbs

Kieran Tierney signed a five-year contract at Celtic last summer

Alan Stubbs believes former club Celtic need to improve defensively in Europe and says Kieran Tierney is their "only top-class defender".

The highly-rated 20-year-old helped Celtic qualify for the Champions League group stage this week.

Centre-backs Erik Sviatchenko and Dedryck Boyata are currently injured and manager Brendan Rodgers has fielded midfielder Nir Bitton in defence.

"The only top-class defender they've got is Tierney," Stubbs said.

"Domestically, Sviatchenko, Boyata, [Jozo] Simunovic are competent, but the Champions League is a whole new ball game.

"[In the Champions League], you need to allow Tierney to go forward, and that puts extra pressure on the two centre-backs and the opposite full-back. That's where the problem lies within.

"Last year when they went into the group stage, people thought they just needed to be competitive and do themselves justice - I don't think anyone thought they were going to come through that tough group.

Stubbs won two league titles, a Scottish Cup, and two Scottish League Cups at Celtic

"But the mindset's changed now, people are potentially looking for Celtic to progress, to see whether that's a measure of Brendan improving in Europe, and to do that, I don't think they're good enough [defensively], apart from Tierney. I really don't.

"That's the biggest area of the team when they go to the Champions League they need to improve."

'Why on earth sell Tierney now?'

Tierney, now a Scotland regular, featured in Celtic's opening three group stage matches last season against Barcelona, Manchester City and Borussia Monchengladbach.

And another ex-Celtic player, John Hartson, believes Tierney's form domestically and on the continent has already attracted bids from Premier League clubs in England, but says the Scottish champions would be foolish to sell this summer.

• Listen: Stubbs and Hartson on BBC's Scottish football podcast

"Why on earth would Brendan Rodgers sell Tierney right now?" former Wales striker Hartson said of the left-back, who has four years of his five-year contract remaining.

"If I'm Brendan Rodgers, I'm saying to [a potential bidder], 'let him have another season here, keep your £25-30m for now, he's got Champions League football, he loves playing for Celtic'.

"Give him another year, 18 months at this level until he becomes more of a man, more established, more mature physically, then in 12-18 months' time, there is no doubt Kieran Tierney will go on and play for one of the top three or four clubs in England, and become one of the best left-backs in Europe.

"I have every confidence in Kieran Tierney to become a top, top player. He is a great player now, but he can only get better."

Hartson played Champions League football with Celtic and helped them reach a Uefa Cup final

Aside from the on-field benefits of retaining one of their prized talents, Hartson believes keeping Tierney for another season, or longer, would only up the price suitors are willing to pay.

"Celtic are looking at, if a bid was to come in now, £20-25m," he asserted. "If Kieran Tierney has another season, if he's outstanding in the Champions League, playing really well against the top teams, in the spotlight, all over the world being looked at by all the top clubs, all of a sudden, next year or the year after, you're getting £45-50m for him.

"That's how inflated these prices are now in terms of everybody paying absolutely massive odds, because the transfers have gone absolutely ridiculous. It's absurd, the numbers clubs are paying now for players."

Simunovic sale 'would be ludicrous'

Celtic had not conceded a goal in five European games prior to Tuesday's 4-3 defeat away to Astana, where they fielded a makeshift central defensive pairing of Bitton and Kristoffer Ajer and won the Champions League play-off 8-4 on aggregate.

Hartson believes Celtic cannot afford to sell Simunovic, the mainstay of their rearguard, who has reportedly attracted interest from clubs elsewhere in Europe.

Simunovic (right) sat out Tuesday's defeat in Kazakhstan due to Astana's artificial playing surface

"We talk about Celtic's defence - they'd not conceded a goal in the qualifiers until Astana away," Hartson added on Sportsound.

"Who is Brendan going to get in now at this stage?

"You cannot sell Simunovic, because you can't leave yourselves short of centre-backs. They can't lose him.

"Boyata is injured, Sviatchenko is injured, you can't play Bitton against the Barcelonas of the world. It'd be ludicrous to sell Simunovic."