"It's technically a dual call-up," Waibel said.
Rusnák's next month will be booked solid, with flights overseas then around Europe. The Slovakia U-21s are drawn into Group A at the U-21 Euro championships along with England, Sweden and host Poland. Slovakia's opening match in the tournament is slated for June 16 against the hosts in Lublin, Poland.
Slovakia then faces England on June 19 in the second match of group play and lastly Sweden on June 22. Waibel said Rusnák is expected to be gone as long as Slovakia's run in the youth tournament lasts.
According to UEFA's U-21 competition format, players are eligible for the U-21 championship if they were born on or after Jan. 1, 1994. Rusnák turns 23 in July.
The attacking midfielder will miss RSL's 2017 U.S. Open Cup intro June 14 at Sacramento Republic FC as well as the return to league play at home June 17 against MLS expansion franchise Minnesota United. While RSL hasn't been able to wiggle out of its ongoing rough patch, Rusnák has been a bright spot in his first few months in MLS. He has three goals and five assists in his first 14 matches with RSL.
RSL will be without its creative midfielder for the next few weeks, but Waibel said the sting should be lessened with the return of Justen Glad, Brooks Lennon, Sebastian Saucedo and Danny Acosta, who helped the U.S. U-20 national team make a run to the FIFA U-20 World Cup quarterfinals in South Korea.
Waibel added that center back David Horst and midfielder Jordan Allen are expected to return to training ahead of the Open Cup match next week in Sacramento. Horst underwent knee surgery in early April and has missed RSL's last 13 matches after starting the first two of the year. Allen's injury-riddled past returned, too, having missed the last 13 matches after picking up a quadriceps injury the second week of the year.
Traditionally a club hit among the hardest in MLS during the summer due to international duty, Waibel doesn't expect the number to be as large as it has in the past. No RSL player was listed on Bruce Arena's 40-man preliminary roster for this summer's CONCACAF Gold Cup tournament.
"The Gold Cup is always an interesting one for us," Waibel said. "We'll see what happens. I don't anticipate losing a lot of the players to the Gold Cup this year, which is good. I hate to say it, but selfishly, I work for Real Salt Lake. I don't work for U.S. Soccer. Every once in a while it'd be nice to not see our guys being gone all summer."