Following his £8million summer transfer from Barcelona, Deco has made a seamless transition to life in the Barclays Premier League with two goals - both brilliant long-range efforts - and two assists in three matches.
Southgate has been named manager of the month for the first time in his career - in the process becoming only the second man, after Stuart Pearce, to win the player and manager awards.
Boro have won six points out of nine so far, beating Tottenham and Stoke at home and only losing at Liverpool due to a Steven Gerrard strike deep into injury-time.
Southgate said: 'I feel it's a great honour, not just for me, but for everybody at the football club.
'It's an honour I very much accept on behalf of myself and my staff because everything that we have achieved so far has been a team effort.
'I think the award is reward for the hard work everybody here put in during July to prepare for the new season.
'We have a really united club here and everybody has pulled together, and in that I include the players as well.
'There's a great atmosphere around the club at the moment and around the town, and long may it continue.'
Southgate is starting his third year in charge at Boro and has guided the club to their best start to a campaign for nine years.
