United are currently languishing in 12th in the Premier League and surrendered a two-goal lead to lose 5-3 to newly-promoted Leicester City on Sunday.
Van Gaal is also facing an injury crisis in an already suspect defence with Jonny Evans, Phil Jones and Chris Smalling all sidelined while Tyler Blackett is suspended, having been sent off at the King Power Stadium.
And when asked if this was the toughest situation of his illustrious managerial career, the Dutchman replied: "At this moment maybe, but you have to see us in a year and not in the one week when we have nine injuries and a red card.
"As a manager you know already it is a big challenge. This club is in a transition and you don't know in advance what are the difficult moments, you have to see, because when we started we won everything, and in very difficult circumstances, now we are again in very difficult circumstances and we can win."
A depleted United will take on West Ham at Old Trafford on Saturday and Van Gaal detailed the nature of the injuries that will keep Evans and Smalling out of action.
"Evans' bone is bruised," he revealed. "Smalling left yesterday's session because he had stiff legs. I won't take risks with players."