United stayed in fourth place but gave renewed hope to its rivals for the final Champion League qualification spot by failing to beat a team in the relegation zone.

Ronaldo hit the post and had a goal disallowed for a marginal offside in a one-sided first half that also saw Bruno Fernandes squander a one-on-one chance and miscue a volley wide from point-blank range.

In the second half, Watford defended better but still gave up chances, including when Anthony Elanga was sent through by a flick from Paul Pogba but blazed wide.

Ronaldo had United's last good chance but met a cross from Fernandes with a weak effort that was saved easily by Ben Foster.

United is two points clear of fifth-place Arsenal, which has now played three games fewer and isn't in action this weekend. Seventh-place Tottenham is five points behind United, having played two fewer games.

Watford remained in next-to-last place and is three points from safety.

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