Koeman opted to rest the Argentinian international from the start at the Estadio Benito Villamarin ahead of a demanding run of games in the coming weeks.
However, with La Blaugrana trailing 1-0 at the start of the second half, Koeman turned to his talisman to haul his side back into the game and Messi responded with an instant equaliser.
“I was talking to Leo yesterday about giving him a break and, if we needed him, putting him on”, Koeman told a post match interview with Marca.
“He came in and changed the game. He has given us a lot of play.”
The veteran striker is almost certain to start against the Basque side as they warm up for their Champions League last 16 tie with Ligue 1 giants Paris Saint-Germain.