Cesar, who took charge of the Slovenian national team in January, used a press conference on Monday to raise his concerns over the Manchester United striker's availability pattern, even as he accepted the legitimacy of the current absence.
Sesko, 22, will miss Slovenia's upcoming friendlies against Cyprus and Croatia in June after aggravating a shin injury against Liverpool earlier this month — a collision that also kept him out of United's final three games of the season.
The current situation Cesar understands. It is the March absence he cannot reconcile.
During the previous international window, United withdrew Sesko from the squad for friendlies against Hungary and Montenegro, citing the need to manage his shin condition. Yet in the weeks either side of that break, Sesko featured in Premier League fixtures against Bournemouth and Leeds United for his club.
"He missed the last assembly in March and then I kind of wanted him to be absent from United for a while, but then he came back quickly and I don't think he missed a game for the club," Cesar said. "But if a player hasn't been in a competitive rhythm for a month, then I don't see the point in him coming and playing at half-strength."
It marks a notable escalation from the Slovenia coach, who had already warned in March that his national team would not be pushed around over the striker's availability.
Sesko has now gone without a Slovenia cap since October 13 despite making 32 appearances for United across the season. His November absence was caused by a knee injury. His March absence was attributed to the shin problem. June makes three in a row.
The player's personal commitment, Cesar was at pains to emphasise, is not in question. Sesko reportedly wanted to report for duty this time. He travelled to Ljubljana for Cesar's first training camp in March despite his injury, and met the Slovenian ambassador to the United Kingdom last week.
United and Slovenia's medical staff reached the joint decision that he should not participate.
"In cases of injuries, the national team receives all the documentation, and then we decide together with their doctors what is best," Cesar said.
The £73.7 million summer signing finished the season with 10 goals in 32 appearances across all competitions for United. The World Cup begins next month — but not for Slovenia, who failed to qualify.
Cesar's frustration, then, has no immediate competitive outlet. But it is clearly building.