And Smalling believes that if United can get to within touching distance before that game at the Etihad Stadium, they will have a chance of lifting the title.
Mourinho, however, did not explain the nature of the deal and said until there is an official announcement Henrikh Mkhitaryan is a Manchester United player and Alexis Sanchez remains an Arsenal forward.
On the latest with Sanchez, Mourinho said: "Nothing new, I think everybody knows that we are there and especially when Arsenal's manager speaks so clearly there is no point in trying to hide or to deny".
In fact, Mourinho's side have now played 504 minutes since they last conceded - coincidentally in the 2-2 draw with Burnley at Old Trafford on Boxing Day. They are expected back at Carrington on Tuesday before their FA Cup fourth round tie at League Two Yeovil on Friday night. United's previous trip to Turf Moor ended in a 2-0 victory and they will hope for something similar this time around as well.
"We know the way they play, they are better than anyone playing that way, we had to adapt to be successful, we adapted well from the defensive point of view".
Even if they can pull level, a very formidable task in the least, City are +19 on United in goal differential.
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Along with Chelsea, United have the stingiest defence in the division and Smalling credited Mourinho's work in forming that solid base following the 1-0 win over the Clarets.
Having failed to register a shot on target in the first half, Martial fired the victor in off the crossbar nine minutes after the break.
"I'm really happy. You like the transfer market, I can say that in this moment I don't see many left-backs better than this Luke Shaw".
"That is the danger in the modern game and the problem I find in clubs", said McClaren, the former England manager and Sir Alex Ferguson's assistant when United won then treble in 1999. Every game is a different story so we have to try and get the three points.
"We've all been there where leads have dwindled and teams have been caught, but we can only focus on ourselves", he added.