United are set to confirm his position as new No 2.
The two Dutchmen arrived in the capital via private jet on Wednesday afternoon. The pair have been working from Holland prior to their arrival in London.
The new United coaching duo are expected to hold a meeting with colleagues over the next 24 hours, says the Daily Mail.
Ten Hag and Van der Gaag could attend United's final game of the season against Crystal Palace at Selhurst Park.
Last week Ten Hag called a vital planning meeting in Amsterdam to which Steve McClaren was summoned - a major indication that the former England boss will join Van der Gaag as an assistant coach.
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