Messi, 33, has scored 18 times in the league this season, which equates to around 34% of Barca's goals in the Spanish top flight. Antoine Griezmann, with six, is Barca's second top scorer, while Ansu Fati, injured since November, is third with four goals.
Barca travel to Sevilla on Saturday with the chance to temporarily move within two points of leaders Atletico Madrid, although they will have played two more games, and level with Real Madrid. Sevilla, meanwhile, can leapfrog Barca into third with a win and will still have a game in hand on Koeman's side.
"It's not just up to the senior players to step forward," Koeman said in a news conference on Friday having previously called on the veterans to take more responsibility.
"To give an example, Leo has scored 18 goals. The rest of the attackers, together, have more or less the same amount. He also needs help.
"The responsibility has to be shared around. Yes, the older players must help the younger players in the squad, but in general it's up to the whole team to step forward."
This weekend's game at the Sanchez Pizjuan is the first of a double-header between Barca and Sevilla, with the sides meeting again at Camp Nou in the second leg of their Copa del Rey semifinal on Wednesday. Sevilla won the first leg 2-0.
Wins for Sevilla could spell the end of Barca's season in March, with the Blaugrana 4-1 down against Paris Saint-Germain in the Champions League, but Koeman doesn't agree he's facing the most important week of his tenure so far.
"There's pressure for all teams, not just us, despite what it may seem [in the press]," he added.
"Being a Barca player or the Barca coach means pressure exists in every game. If you don't win, if you lose games, then the coach always to blame. I accept that responsibility."
Koeman's ended prematurely when he got a nosebleed for the second time this season during in a news conference.
A club source told ESPN he's fine and explained he takes blood thinners after suffering a heart problem last year and that they're the cause of the nose bleeds.