In the final of the Spanish Super Cup, which Barcelona would go on to lose in extra-time, Messi was sent off for the first time in his career with the club after slapping Asier Villalibre on the back of the net after a coming together between the pair.
It was a sign of Messi's frustration in that game, and probably his frustration at the entire season, that he would lash out so rashly.
He has now served his two-match suspension after that dismissal, having missed last week's Copa del Rey win against Cornella before the league game against Elche, which was also won 2-0.
However, with Barca still struggling in La Liga 10 points adrift of leaders Atletico Madrid, Messi might be kept fresh for the match against Athletic this weekend.
The Blaugrana should have enough to beat Rayo without Messi, but as Real Madrid and Atletico can confirm this season - nothing is guaranteed in the Copa del Rey.
If Messi is rested, he'll be available from the bench if needed.
Barcelona aren't blessed with a huge amount of options in attacking areas, but Francisco Trincao will likely be given a chance to impress, potentially at the expense of Ousmane Dembele.
Miralem Pjanic will be given a start which will mean Sergio Busquets dropping to the bench.
Samuel Umtiti and Ronald Araujo both played well at the weekend and could retain their places with Clement Lenglet kept fresh for Athletic.
The same could be said for Sergino Dest, which means another start for Oscar Mingueza.
In goal, Neto could come in for Marc-Andre ter Stegen.