In the 2019/20 campaign, the Blaugrana failed to win a trophy for the first time in 12 years, with Real Madrid winning La Liga and Bayern embarrassing them in the Champions League.
It was after the loss against the German champions which led to head coach Quique Setien being sacked, to be replaced a few days later by Ronald Koeman.
Koeman has been tasked with revamping an ageing squad, and his two main transfer targets are believed to be Gini Wijnaldum and Memphis.
Memphis will be a team-mate of Messi's should he sign for Barca, with the Argentine set to stay in Catalunya after previously announcing an intention to leave.
"There's always a question mark," Van Gaal toldTuttoMercatoWeb with regards to Memphis' talent.
"Depay is an interesting player, especially when he's free to play, but we have to consider adapting to a new culture and a new team, and the first year is always the hardest. Look at [Matthijs] De Ligt or Frenkie de Jong.
"Depay has talent, but he's not Messi or Cristiano Ronaldo."
As for Koeman, Van Gaal believes his compatriot has the ability to do a good job, but worries that the Messi saga will hang over his tenure.
"I think he is a good coach, a former Barcelona player and this gives him more credit. It won't be easy because you can never know about the Messi problem," he added.
Barcelona have already sold veteran midfielder Ivan Rakitic this summer, with Luis Suarez and Arturo Vidal two players likely to follow him out the door.
Memphis starred for PSV before earning a big-money move to Man Utd with Van Gaal at the helm. After failing to impress, he signed for Lyon to become their star player and captain.