The Magpies lined up in an attacking 4-1-4-1 formation, and paid the price as the Hornets overran them in midfield and took the lead through two Odion Igahlo strikes.
“We will have to look at the game again,” said McClaren.
“We were at home and we needed to be positive so we put a team out there to be positive to go and win the game.
“All the way through that’s what we did. So much so it almost got us back in to the game but that’s (also) what got us into trouble in the first place.”
Newcastle were second best all over the pitch, and look in need of a major rethink if they are to get their season back on track.
McClaren’s experiment suggests the coach is yet to work out his best system.