They had each made such an impression in their first season at the club that they were established as the emblems of Neil Lennon's side: young, gifted, ambitious, and central to the team's potential. They were already vital, and the wonder was how long the club could retain them.
They have not all been eclipsed now, but Kayal hasn't featured since damaging his ankle last December, while the prominence of Izaguirre and Hooper in Sunday's emphatic 3-0 win over Rangers at Celtic Park was a timely reminder of their worth.
The champions could be tempted to accept reasonable offers for both players as the profits would be reinvested in the team and Lennon already has other options at left-back, most notably Charlie Mulgrew, while a tall, strong, aggressive centre-forward is already near the top of the Celtic manager's list of summer targets.
Even supporters might not agonise over the departure of either player, at least not as much as they would have last summer. Izaguirre was the Professional Footballers' Association Scotland's player of the year last season, but an ankle fracture has restricted him to only 11 appearances this campaign, keeping him on the sidelines from August to January. Hooper has played more games, and scored more goals, in this campaign, but his form dipped enough in recent months for many Celtic fans to question why he remained in the starting line-up ahead of Anthony Stokes.
Izaguirre tore up and down Celtic's left-flank on Sunday and looked to have recovered the best aspects of his game – adventurousness, drive, dynamism – although he was aided by the manager's switch to a 3-5-2 formation.
If there was any criticism of Izaguirre last season, it was of his defensive instincts, and Lennon has often remarked ruefully about the pangs of alarm he feels when he sees Izaguirre setting off on another of his forays upfield but, at wing-back, he is less exposed.
Some managers consider attacking full-backs as critical in the modern game, when so much of the space infield is compressed and overrun, and several Barclays Premier League clubs intensively scouted Izaguirre last season.
He might not generate the same premium fee as he would have done last summer, but he has proved his fitness again. With Mulgrew, Joe Ledley, Adam Matthews and Andre Blackman all having played at left-back this season, Lennon is not short of options for that area, and he has already signed Jaroslaw Fojut, a 24-year-old Polish centre-back, on a pre-contract agreement, adding to his defensive options.
Hooper has been able to play more regularly this season, and some of his link-up play with Stokes has been vividly effective. At his best, he is incisive, accelerates quickly past defenders and possesses a powerful finish.
Southampton have long been keen on Hooper, and can now offer the striker Premier League football, as well as being able to afford an offer of around £8m now that they have been promoted from the Championship.
Lennon values Hooper, and retained his faith even when the striker was performing as though subdued by his own self-doubt. The Englishman has scored 24 goals this season and would be difficult to replace, since Stokes is less able to adapt his game to suit the needs of the team.
Georgios Samaras has all the attributes to play through the centre regularly, but is inconsistent and also offers an effective outlet on the left of midfield. Celtic need to sign a top-level striker, though, as well as a centre-back of similar quality and a goalkeeper. But bringing a new forward to the club would surely demand that Lennon sell one of his four main attackers, including Mohamed Bangura, anyway.
The squad is well-stocked with central midfielders, but Kayal has only been surpassed in the quality and presence that he brings to that area of the field by Victor Wanyama. The two would form a daunting partnership, and Kayal is capable of setting the tempo of Celtic's play with the crispness and range of his passing.
Reputations are vulnerable to circumstances, as the fate of Kris Commons has shown this season. He was brilliantly assertive in his first six months at the club, and then fell out of form, favour and shape, before recovering some of his virtuosity in recent weeks. Against Rangers, Commons was irrepressible, and so confirmed the restoration of his status as a player who still has much to offer the side.
This is the dilemma for Lennon, how to judge the long-term worth of his men against their current form.
Izaguirre, Hooper and Kayal were once considered irreplaceable, but it is conceivable that only one could be at the club next season yet the team would still have improved by shrewdly investing the transfer income.
Lennon can upgrade his side because it now has strong foundations.
