Manchester United boss Louis van Gaal's halftime substitutions for defeat to FA Cup opponents Arsenal have been explained.

The Manchester Evening News says Luke Shaw's chance was enforced, however Ander Herrera's withdrawal was tactical.

It is understood Shaw, who was sidelined for the recent wins over Sunderland and Newcastle, was not 100 per cent fit and got removed as a precaution.

Van Gaal was in deep conversation with midfielder Michael Carrick as he strode down the touchline at the interval and he was swapped with Herrera for the second-half.