Gary Caldwell needs little reminding about how fine the margins usually are between success and failure in the Champions League. The Wigan centre-half, currently nursing a hip injury, was back at his old stomping ground in the east end of Glasgow on Tuesday as a combination of his former team-mate Scott Brown's impetuosity and some spurned chances led Celtic to go down to Barcelona to leave themselves bottom of Group H.
The Parkhead side still have it in their power to write an alternative script - they now face a double- header against Ajax while AC Milan have not impressed - but Caldwell recognises certain echoes of the club's fortunes under Gordon Strachan, when the accumulation of elements enabling them to qualify so impressively in 2007/08 (including late goals in home wins against AC Milan and Shakhtar Donetsk) failed to build momentum as the club were pipped to a consolation Europa League place by Danish minnows Aalborg 12 months later.
"It is a very fine line and obviously Celtic are treading that right now," said Caldwell. "The year we qualified we scored last-minute goals here against Milan and Shakhtar and you just needed that bit of luck. The next year we didn't get it - we drew here in our opener against Aalborg, but we battered them that night and just couldn't get that goal. Then we went over there and were leading with 10 minutes to go, but conceded two to lose 2-1."
"The Champions League matches are such big nights on the calendar for Celtic, especially without Rangers in the Premiership," he added. "To have that, and to have it after Christmas, is such a massive boost for the whole club. I remember the year we went out we felt our season went really flat. It is such a great competition that you want to stay in it as long as you can."
The home-and-away double- header is often defining in a Champions League group stage and rarely more so than for Celtic this year. The Parkhead side feel they should have points on the board and will be hopeful of getting them, particularly at home, against an Ajax side who crashed to a 4-0 defeat by rivals PSV Eindhoven last month and are part of an eight-strong bottleneck at the top of the Eredivisie. If that assignment goes wrong, however, then the campaign will be in tatters. "It is still all to play for," said Caldwell. "They have Ajax at home first, and they really have to win that game. That means they can go to Amsterdam where a point would be good, but they would actually probably fancy themselves to win over there. So the next game is massive and if they don't get maximum points from that, then I think it is probably all over for them."
If it wasn't for that trademark shaggy mane and beard, Caldwell might have struggled to recognise Georgios Samaras, such is the transformation the Greek has undergone in recent times. Although there were no goals or assists, rarely has Samaras appeared more totemic to the cause than in midweek, a far cry from the hit-and-miss figure who featured back in Caldwell's day.
"He is a fans' favourite now, he was never that in my day," said the 31-year-old. "He seems to have the bit between his teeth a bit more. When he got the armband he tried to drive Celtic forward - there was a real willingness from him which I didn't see a few years ago. He has probably matured and become a more rounded player. I think he and Kris Commons, the forward players, will be the key in these Ajax games. Against Barca it was about shutting up shop, but against Ajax they are going to have to attack, they are going to need the crowd to push them forward and the attacking players are going to have to be more of a threat."
Neither could Caldwell fail to be impressed by his first glimpse at the club's burgeoning central defensive partnership of Virgil van Dijk and Efe Ambrose. It was late in the game, and Celtic were down to 10 men, by the time Barcelona ever really cut them open - which is a fair accolade.
"It is the first I have seen Van Dijk live and I thought he was excellent," said Caldwell. "He looked very composed on the ball, big, powerful, pretty quick and most of the night he and Ambrose were pretty comfortable. They look like they complement each other well and both can play on the ball, which I think you need in the modern game. Against Barca they didn't have a centre-forward to mark at times, which can be difficult - you want to go looking - but they held their position well. The middle of the goal was locked up and the goal they scored was a breakaway really - they got done pushing forward a bit too much."
As for his own situation, Caldwell is back running on grass at the Lancashire club this season, but still rates himself about "six to eight weeks" away from a full return to the rigours of Championship football. That is why he was omitted from the Scotland squad which takes on Croatia in nine days' time, and why he will also miss the USA friendly, but he is still a key member of Gordon Strachan's group. Getting back into the team might not be so easy, however, with Grant Hanley having taken massive strides in another growing partnership, with Russell Martin.
"They've been great, but the whole team has been excellent," said Caldwell. "Since the gaffer has come in, maybe in the first games we didn't do so great, but I always knew we would get it right and that has been the case. Performances have got better and with that the results are coming, but I know they can get better still. I played with Grant a few times in the gaffer's first few games and I thought he was excellent, somebody with real potential to have a bright future, and he has done great in the games - and Russell has helped him with his experience."
"As a young player there are always going to be errors, it is about learning from them and progressing and Grant is doing that," he added. "I was down at the England game where we didn't win, but the performance was good so there was still a good feeling from that game that we continued into the next double- header. It is important that we keep that going against Croatia, that we finish the campaign on a high to prepare us for the next one."