"He has a big arm," Watson said.
Although a final decision has not been made, general manager Dayton Moore said that Castaneda, who is signed to a Minor League contract, might be invited to the Major League camp as a non-roster invitee to Spring Training.
"We're looking at that. We'll see how he finishes up in Mexico," Moore said.
In winter ball, Castaneda has pitched in 17 games for Culiacan, with no record and a 1.33 ERA. He has 15 strikeouts in 20 1/3 innings.
Castaneda, from Torreon, attended the MLB-supported baseball academy in Mexico at the same time as Soria, now the Royals' closer. Like Soria, he's been largely overlooked.
Spring Training could change things, but right now Castaneda is projected as a Double-A reliever next summer.
Two other players from outside the organization, shortstop Luis Hernandez and third baseman Corey Smith, have signed Minor League contracts with the Royals and were invited to the Major League camp.
Hernandez played in 66 games for the Baltimore Orioles over the last two seasons, batting .264. Last season, he hit .184 in 57 games for Triple-A Norfolk. He was originally signed in 2002 by the Atlanta Braves.
"He's young  and is an above-average defender at shortstop and second base," Moore said. "A switch-hitter, he knows how to play and gives you some depth in the middle."
Hernandez could compete with Tony Pena as a backup shortstop.
Smith, 26, is a right-handed batter who banged 26 homers and had 80 RBIs last summer for Double-A Arkansas in the Los Angeles Angels' organization.
The Royals also re-signed two of their own Minor League free agents, pitchers Matt Wright and Tim Hamulack, and invited them to Major League camp.
Wright, a right-hander, was 4-10 with a 5.47 ERA in 27 games (23 starts) for Triple-A Omaha. Hamulack, a left-hander, was 0-1 with a 1.96 ERA and 23 strikeouts in 23 innings in 18 relief outings for Double-A Northwest Arkansas.
Also signed to a Minor League contract was left-hander Carlos Sencion, who was 6-4 with a 4.72 ERA last season for Double-A Mississippi in the Braves' organization.