The Scottish champions signed six players ahead of the European deadline; they came after they failed to reach the Champions League; Martin O'Neill has made 10 summer signings so far; watch St Mirren vs Celtic on Saturday, live on Sky Sports from 7.30pm
Celtic made it six signings in little over 24 hours ahead of Wednesday night's UEFA registration deadline.
The Hoops brought in four loan players ahead of their 3-0 win over Aberdeen and confirmed the arrivals of 19-year-old midfielder Joel van den Berg from PSV Eindhoven and Swiss right-back Jordan Lotomba soon after the game.
Danish midfielder Oliver Sorensen and Chelsea striker Shim Mheuka made their debuts off the bench in the Scottish Premiership clash.
Wolves goalkeeper Sam Johnstone was also among the substitutes while a second 18-year-old Chelsea player, left-back Landon Emenalo, also signed but is several weeks away from fitness.
Van den Berg then arrived on a four-year deal while 27-year-old Lotomba has signed until the end of the season.
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Both signings were completed just ahead of the UEFA registration deadline of 11pm on Wednesday.
But, with limited space for overseas players in their Europa League squad, Celtic could make further signings before the Scottish Premiership deadline of 5pm on Thursday.
The Hoops have been linked with a move for Torino midfielder Gvidas Gineitis.
Celtic freed up further space in their squad on Tuesday night with the permanent departures of Johnny Kenny and Reo Hatate to Preston and Burnley respectively.
Speaking after the win over the Dons, manager Martin O'Neill said of Celtic's late arrivals: "I'm very pleased that they're in the door. I think that they'll definitely add something to us.
"Honestly I am pleased, the transfer window is there and I'm pleased that we have done what we have because there's quite a few who have left the football club.
"Reo's gone and Johnny Kenny has gone, I wish them all the best and so we need people in. Even if they miss the European deadline, if there is a possibility of getting someone else in, I would definitely want to do that if we can."
The six new arrivals in September follow the earlier summer signings of Camilo Duran, Kasper Hogh, Mika Baur and Haissem Hassan.
