No-one had to be Hercule Poirot to recognise what was missing from Celtic’s summer player shopping.

The word acreage devoted to the absence of any seasoned professionals among the nine purchases made by the Scottish champions – plenty of it offered up on a consistent basis by Brendan Rodgers – could stretch to the outer reaches of the galaxy. Yet the specifics can still be eye-widening as the subject has inevitably become a cause celebre in the wake of the 2-0 defeat away to Lazio in the Champions League on Tuesday that summed up another deflating and fruitless tilt in European competition. The fourth such sortie in five when the club have been consigned to bottom place in their group section, a product of claiming only two wins from the 29 contested in club football’s most prestigious tournament in over a decade. The second such victory, oh, just the six years and 15 outings ago …

When the Northern Irishman first pitched up in Glasgow in June 2016, he immediately sought out tried and trusted players. Three of his first four signings – Scott Sinclair, Dorus de Vries and Kolo Toure – were at advanced stages of their careers with hundreds of senior games under their belts. In his successor Neil Lennon’s opening transafer window, signed first-up as unquestionably a statement signing was 26-year-old centre-back Christopher Jullien, acquired for £7million and with experience at four different clubs under his belt. In turn, two summers ago, six of the first nine players Ange Postecoglou recruited had passed their 26th birthdays in Kyogo Furuhashi, Carl Starfelt, James McCarthy, Joe Hart, Josip Juranovic and Giorgios Giakoumakis. Or, if you want to frame that another way, almost as many players in that age bracket as to be found among Celtic’s previous 50-plus acquisitions.

Callum McGregor and Mikey Johnston prepare in freezing conditions for Celtic's match against St Johnstone.

No new manager at the club in almost a decade then has found themselves embarking on a refresh with only a raft of players in the early 20s and, elder stateman in 23-year-old Luis Palma apart, a cohort in which each was without 100 senior appearances to their name. That was the curious situation for Rodgers as almost £18m in fees was invested in eight players who adhered to the club’s trading model of buying promising young players to polish up and sell-on for a profit. Not a single one of them could possibly have been a personal pick. An individual over whose attributes he would have had sufficient in-depth knowledge to convince him that they could immediately front up and slot in. A very different situation from the opening season of his first stint. Moreover, with the departures of Aaron Mooy, Juranovic, Giakoumakis and Starfelt, he was plunged into a Champions League campaign with the treble-winning squad shorn of four of the more established players to have appeared in last year’s equally exasperating campaign in the blue riband tournament. From Rodgers’ subtle, and not so subtle hints, it is patent that there will be no repeat of this scenario for future such assaults.

“There’s no doubt that experience is important,” said the Celtic manager. “You cannot just have a team of really young players. You’re never going to extract that potential out of them unless you have that experience beside them. Also, there is the number of games you play. If you solely rely on young players for game after game after game then they don’t have the games in their legs. You’ll find they will break down. So for a number of reasons … [we need a certain profile of player]. Listen, every club will look to bring in young talented players and that can be a business model – you progress them and then they fly and the club gets the money. But every player can’t be that.

“And if you’re bringing in players of quality, then you need to know what they bring. It’s balance because I enjoy working with young players and seeing them grow and develop. But, equally, I love working with senior players who can help bring them through. Being here the first time, Scott Brown was a massive influence on Callum McGregor when he was 32. Kolo Toure was a great role model for Dedryck Boyata and the other centre-halves here in helping them get through games. Scott [Sinclair] was a proven player and I knew what I was going to get with him. He was going to bring speed and goals to the team. You can’t dismiss that. You definitely need that experience in games. We have had one window in which a number of players were over the line to come in but hopefully in the coming windows we can get the squad to a balanced level where it’s a mixture of experience, youth but always with quality.”

Rodgers told broadcasters on Friday that he considered the Celtic squad was four time-served and capable performers short even before Jota and Starfelt made their exits. He stepped back from an actual number when asked to expand on the comment but, with 32 senior players, twice as many players could make way in the January window to allow him to shape up his pool in the fashion he believes is most beneficial. “Ideally we’d have a squad of 25 players, 22 with three goalkeepers, “ he said. “In which case there will also be an opportunity for young players to come through as that’s always my first look. Throughout my career you’ll see a whole host of young players that I’ve always put in, guys who are 17, 18, 19 or 20. I’ve always looked to that enthusiasm and rawness of a player to bring in. So I’ve always looked from within. If it’s not here for whatever reason then you need to bring in from the outside.”

That need was there before the season began. And wasn’t met. In the next two windows, it can be presumed this would lead to corrective measures.

'They will break down': Celtic plan for corrective measures for imbalanced squad as days of Kolo Toure referenced