Midfielders Memphis Depay and Morgan Schneiderlin have been sold in January to Lyon and Everton respectively and so far no players have been brought in.
A move for Benfica's Victor Lindelof appeared to be close earlier in the month but failed to materialise.
Manchester United's FA Cup player ratings against Wigan
Sun, January 29, 2017
Express Sport brings you Manchester United's player ratings from their FA Cup fourth round win over Wigan Athletic
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Sergio Romero – 7.1
But according to The Sunday Times, Mourinho wants to bolster his squad in the closing days of the transfer window.
It is claimed the United boss is concerned about his side's busy domestic and European schedule.
anchester United manager Jose Mourinho
An approach for AS Monaco's Tiemoue Babayoko has been made with Mourinho's preference being to add a midfielder.
But Monaco and United have failed to agree a fee with the French side reluctant to sell such a key player.
Executive vice-chairman Ed Woodward reportedly believes Lindelof and Babayoko can be signed in the summer for cheaper fees.