The Central Americans, who last qualified for the finals in 1982, face Chile on June 16 in Group H, before pitting themselves against European sides Spain and Switzerland.
Boss Reinaldo Rueda will be boosted by the news that his star midfielder will now be available, having seen him stretched off during their last outing.
Palacios left the field after 67 minutes as his side slumped to a 3-0 defeat against Romania on June 5.
Subsequent tests revealed a muscle injury, but the 25-year old remains upbeat about making his team's opening match.
"I suppose I pulled an abductor but I feel much better. Thank God it's nothing serious," Palacios told La Prensa.