Domenico Tedesco said Belgium's lengthy injury list was to blame after they were stunned by Israel in their final Nations League group-stage game on Sunday.
Belgium were beaten 1-0 in a game played in neutral Budapest, having also lost to Italy by the same scoreline three days earlier.
Tedesco's team were already unable to finish in the top two of Group A1 and make next year's quarter-finals, and they must now face a Group B runner-up in a promotion/relegation playoff.
Belgium were hit by something of an injury crisis during this international break, which captain Kevin De Bruyne opted to skip as he recovers from a hamstring injury.
Romelu Lukaku was then among five players to withdraw from their squad on Saturday, while both Leandro Trossard and Zeno Debast were substituted with injuries on Sunday.
"Some players played 90 minutes three days ago, and tonight they were somewhat compelled to play another 90 minutes," Tedesco said afterwards, as quoted by UEFA.com.
"Overall, the lessons learned were also part of the plan; it was about getting to know many players, as we will be there in March.
"I believe we had 21 injured players in the previous qualifying stage, and today we lost Leandro Trossard and Zeno Debast to injuries.
"We had a team that had never played together, never trained together. We started well, but ultimately, under really difficult circumstances, this is not the result we hoped for, and that's why we are very disappointed."
Despite attempting 649 passes in Sunday's game, their most in a single match in this edition of the Nations League, Belgium only managed 12 shots to Israel's 10.
Their total expected goals figure of 0.91 xG, meanwhile, was only marginally higher than the 0.77 created by Israel, who saw Yarden Shua come off the bench to net an 86th-minute winner.
Despite the loss, Tedesco has seen enough to believe his team will remain in the top tier of the competition when March's playoffs roll around.
"We will remain in League A, I am sure of it, because we know how to play in such matches," he said. "This will be important; that is our goal, as is qualifying for the World Cup."