Mackay-Steven's Celtic career got off to the perfect start on Wednesday night, as he scored the opening goal in a 3-0 win over Partick Thistle.
Duffy is expected to be more of a player for the future, but manager Ronny Deila does gain another winger to his squad this week.
Ghana's Africa Cup of Nations final defeat to Ivory Coast means that summer signing Mubarak Wakaso will be back among the Celtic squad having been away for around a month.
The 24-year-old arrived in Glasgow last summer, penning a season-long loan deal, and fans were rightly excited given his reputation during spells with Villarreal, Espanyol and Rubin Kazan.
The pacey winger featured for Ghana at the World Cup in Brazil last summer, but has struggled to make an impact so far, playing just ten games and scoring only once in all competitions.
Wakaso is essentially playing for his future in the second half of the season, and his path to the first team has been somewhat blocked by the arrival of Mackay-Steven.
He now needs to hit the ground running upon his return, with Celtic vying for success on four fronts; the league, the Scottish Cup, the Scottish League Cup and, of course, the Europa League.
Time is running out for Wakaso to make an impact at Celtic, but after a promising tournament with Ghana, he may be hitting form at the right time as he bids for a permanent deal under Deila.