Messi's situation at Camp Nou continues to dominate the headlines following a summer in which he tried to force a move away from the club, only to reluctantly perform a U-turn and commit to staying for the 2020-21 season.
Barca have lost three and won only one of their last six La Liga games to leave themselves 12th in the table, already 12 points adrift of the early pace-setters Real Sociedad.
However, they have a perfect record so far in Group G of the Champions League, and Koeman insists that his side's position as leaders of their group is the reason behind Messi's omission for the trip to Ukraine.
"We've got various players out due to injuries and we've decided that we won't be taking Leo or Frenkie [de Jong] because the situation in the Champions League is quite comfortable for us with the nine points that we've got," Koeman told reporters.
"And these two players need to rest because they've played a lot of matches. So we think this is a good moment to rest them."
Messi has six goals and four assists in 11 games across all competitions this season, including three goals and two assists in his three Champions League outings. body check tags ::