[17] His start to the 2020−2021 swim season in late 2020 along with his performances before competition shutdowns related to the COVID-19 pandemic in 2020 earned him the Swammy Award for "NCAA Male Swimmer of the Year".

[27] At the US Olympic Trials held in Omaha, Nebraska, and postponed to June 2021 due to the COVID-19 pandemic, Casas competed in two events.

[28] In the 100-meter backstroke he swam a 52.76 in the final, finished in third place behind Ryan Murphy and Hunter Armstrong, and did not make the 2020 US Olympic team in the event.

[34] On October 21, he announced an intermediate step in his decision making, remaining at Texas A&M to train with the professional group of swimmers as he prepared for the 2021 Short Course World Championships.

[35] As part of his racing preparation for the World Championships, Casas swam a time of 19.51 seconds in the 50-yard freestyle and a 1:42.96 in the 200-yard individual medley on the first day, November 17, of the 2021 Art Adamson Invitational, choosing to compete in exhibition unattached to any school.

[38] Casas entered to compete in all the individual backstroke events, 50-meter, 100-meter, and 200-meter distances, at the 2021 World Short Course Championships in Abu Dhabi, United Arab Emirates in December.

[40] On the first day of competition, Casas tied Lorenzo Mora of Italy with a 50.40 in the same prelims heat of the 100-meter backstroke and both swimmers qualified for the semifinals ranked third overall.

[49] In the morning prelims session on day three, Casas qualified for the semifinals of the 50-meter backstroke ranking third with a 23.23, less than one-tenth of a second behind second-ranked Gabriel Fantoni of Brazil.

[51] For his first race of the evening session, Casas tied Lorenzo Mora for fourth rank in the semifinals of the 50-meter backstroke with a time of 23.13 seconds and qualified for the final.

[52] In his second and final race of the evening, Casas split a 23.16 on the lead-off leg of the 4×50 meter mixed medley relay to help win the silver medal in 1:37.04.

[2][3] At his first meet with the University of Texas, the 2021 Eddie Reese Invite, Casas swam a 19.28 in the 50-yard freestyle, a 1:11.0 in the 150-yard backstroke, and a 1:12.0 in the 150-yard butterfly.

[74] On day four, he swam a 1:59.86 in the prelims heats of the 200-meter individual medley to advance to the final ranking third with his time being 0.79 seconds faster than fourth-ranked Chase Kalisz.

[78][79] The second day of the 2022 US International Team Trials in Greensboro, North Carolina, Casas ranked first in the prelims heats of the 200-meter backstroke with a personal best time of 1:55.57, qualifying for the evening final.

[84][85] For his fourth event, the 100-meter backstroke on day four, Casas ranked third in the prelims heats with a time of 53.31 seconds, qualifying for the evening final.

[89][90][91] In the semifinals, he went out faster than in the prelims, lowering the first half of his 200 meters to a time of 55.94 seconds, and came back slower to finish in 1:56.90 and qualify for the final ranking sixth.

[92] He swam a personal best time of 1:55.35 in the final to win the bronze medal, finishing 0.19 seconds behind silver medalist Luke Greenbank of Great Britain.

[99] At the 2022 Swimming World Cup stop in Berlin, Germany, his first FINA Swimming World Cup, Casas won two gold medals, the first in the 200-meter backstroke with a time of 1:50.02 and the second in the 100-meter backstroke with a 49.54, which was less than one-tenth of a second faster than silver medalist Thomas Ceccon of Italy.

[100][101] Across all of the events he competed in for the first stop, including those he did not medal in, he ranked seventh amongst all male competitors with a score of 48.9 points.

[102] On the first day of competition at the stop in Toronto, Canada, Casas won the gold medal in the 200-meter backstroke with a 1:48.99, which neared his personal best time of 1:48.81.

[105][106] For the final day, he won the gold medal in the 100-meter backstroke with a World Cup record and personal best time of 48.84 seconds.

[110] By the end of World Cup competition he brought his score up to 164.9 points, ranking fourth overall amongst male competitors.

[114][115] The following day, he swam a 22.98 for the backstroke portion of the 4×50-meter mixed medley relay in the preliminaries to help qualify it for the final with a time of 1:36.83 and overall first rank.

[130] Legend: h – preliminary heat; r – relay 1st leg The following medals Casas has won at Swimming World Cup circuits.