United are fifth in the standings, four points outside the top four and with three games left to overhaul either Manchester City or Arsenal (67 points) for the final berth in Europe's elite club competition next season.
"Expectations are too high," the Dutchman said at United's player of the year award ceremony.
"We are in a period of transition. It is not so easy. We have to meet the expectations of the biggest club in the world."
The 64-year-old, who has come under scrutiny for the side's uninspiring style of play despite a host of high-profile signings, said constant speculation in the media over his future had undermined his relationship with the players.
"When the media is writing for six months I am sacked already... I can cope with that. It is not new for me. For my players, it is not so easy," he added.
Goalkeeper David de Gea was named the club's player of the year for the third consecutive season, while the first-team squad voted defender Chris Smalling as their player of the season.
(Reporting by Nivedita Shankar in Bengaluru; Editing by John O'Brien)
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