The transfer window reopens in the new year, and players can also agree pre-contract agreements.
Mourinho encourages a counterattacking style of football.
United needs to be more clinical in front of goal, said the manager.
Mourinho sold Mata from Chelsea in 2014.
Mourinho wants more noise from the Old Trafford crowd.
United has now gone 11 Premier League games unbeaten.
United toiled for most of the game in east London.
United currently sit in eighth place in the Premier League.
Mourinho has been under pressure following a poor run of results.
Rooney has been dropped from Mourinho's first team.
United dominated but could not put the Premier League struggler away.
Mourinho took over at Manchester United in the summer.
Juan Mata had appeared likely to leave the club.
Juan Mata does have a future at Old Trafford.
Juan Mata can only play one of every three matches.
Juan Mata says reports of his demise should not be believed.
Bringing in new players requires the Portuguese to trim his squad.
Juan Mata is almost certainly on his way out of Old Trafford.
As pre-season approaches, these are the deals the new manager needs to close.
Two big names could leave Old Trafford this summer.
The Manchester United manager has been bemoaning a lack of depth in his squad.