The 31-year-old picked the ideal time to score a first career goal as he scored his side's second of the game in the 95th minute to advance into the second qualifying round at the expense of 10-man Banants of Armenia, despite losing 3-2 on the night.
Leading 1-0 from the first leg, Santa Coloma made a solid start when they went ahead through Ildefons Lima after 20 minutes.
Garegin Mashumyan's goal and Rumyan Hovsepyan's brace then turned the tie on its head only for the game to take another twist when Gevorg Nranyan was shown a second yellow card eight minutes after the hour.
Just when the hosts looked to have held on, Casals kept his cool to fire home the crucial away goal that sent his side through.
Lincoln Red Imps were denied their first ever European victory as they crashed out to HB Torshavn in the Faroe Islands 6-3 on aggregate.
The first nation from Gibraltar to feature in the competition since the country was given UEFA membership status last year, Lincoln came into the game level at 1-1.
A disastrous first half saw them trail 3-0 at the break, but the visitors fought back - George Cabrera and John Paul Duarte scoring to give Red Imps hope.
Goals in the last 10 minutes from Levi Hanssen and Frodi Benjaminsen put paid to their chances, however, with Red Imps left to focus on domestic duty - namely winning the Gibraltar Premier Division for a 13th successive season.
Estonian champions Levadia enjoyed a simple progression into the next round as they hit seven past La Fiorita on home turf to win 8-0 on aggregate.
Igor Subbotin's missed penalty mattered little as he scored twice along with Artjom Artjunin, with Aleksandr Kulinits, Omar Elhussieny and Toni Tipuric also on the scoresheet.
