[5] He made further appearances early in the 2017–18 season, including a UEFA Champions League match against Paris Saint-Germain,[6] but then suffered a knee injury in late September.

[7] On 16 March 2018, Ralston joined Dundee United on an emergency loan until the end of the 2017–18 season, a side challenging for promotion from the Championship up to the Premiership.

[10] Upon his return from the St Johnstone loan, Ralston played in only one game for Celtic during the following season, coming in a 0–0 draw against Livingston when several squad regulars were missing due to a Covid-19 isolation issue,[11] and when centre-half Kristoffer Ajer then loanee Jonjoe Kenny were selected at right-back ahead of him following the sale of Jeremie Frimpong, it appeared his time with the club would be coming to an end.

[16] It has been assessed as one of the most important moments of the season for Celtic as they went on to win the 2021-22 Scottish Premiership title, as the result kept them within touching distance of Rangers who were four points ahead in the table at that time.

[17] At the end of 2022, Celtic signed Canadian Alistair Johnston who quickly became the first-choice right-back, with Ralston's role becoming that of a dependable back-up.

[26] In November 2021, he was called up to the senior squad for the first time ahead of a 2022 World Cup qualifier against Denmark, as a replacement for the suspended Nathan Patterson.