The strikers who have been brought in to replace Gary Hooper up front have not - not yet anyway - filled his boots.
But Virgil van Dijk has certainly filled the void left by Kelvin Wilson in defence.
In fact, the young Dutch centre-half has proved to be a far better acquisition than his predecessor.
Kelvin took his time to settle at Parkhead, but Virgil has proved to be a great catch straight away.
He is solid defensively, he is good on the ball, he is big, he is strong, he is quick and he has scored good goals.
He has got all the attributes that he needs to enjoy a very successful career.
Sometimes you get young players like that who fail to show it during matches, but that does not apply to this boy.
When a foreign player joins Celtic and excels it tends to generate a lot of interest in clubs in England and that is what has happened with Virgil.
He has been linked with moves to several Barclays Premier League clubs in recent days - including Arsenal and Manchester City.
It looks like the Scottish champions could sell on the player, who cost them in the region of £2.5million, for a tidy profit.
They have certainly managed to do that in recent years with the likes of Gary Hooper, Ki Sung-Yueng, Victor Wanyama and others.
But there comes a stage when Celtic have to say enough is enough. And that time may be now.
I don't think they can keep accepting offers whenever a club comes in for a player - even if they can make millions of pounds on the transaction.
If the Scottish champions keep buying and selling then they will be unable to get any stability.
Virgil has only just arrived here. He has become a key player in the side. The last thing Celtic need to do is to sell him.
Of course, every player has a price. But I do wonder if Celtic are sometimes selling too readily.
In the case of Van Dijk it could be time for the club to come out and say: "We are not going to listen to offers."
Selling him should be the furthest thing from their minds as they try to build another successful team.
If he keeps playing the way he has been then it might be something they have no choice but to consider.
But the player has given no indication that he wants to leave. He appears to be very happy with his lot at the club.
The only thing that may be an issue is that he has not been called into the Netherlands squad and the World Cup in Brazil is fast approaching.
Virgil has played for his country's Under-21 team before, but he has not featured for his senior national side.
The Dutch are a top quality international team and will go to South America in the summer with hopes of a triumph. He will do well to break into that set-up.
But if he keeps playing well for Celtic then it is not inconceivable that he will get the nod. So why not try to hold on to him for a couple of years?
Efe Ambrose has spoken of his desire to see the current Celtic squad stay together, and even be strengthened, over in Turkey this week and I can understand what he is thinking.
Celtic cannot keep chopping and changing their squad every season and expect to enjoy success domestically and in Europe.
I think the comings and goings in the summer have shown us that it is not always easy to replace your top players, although Teemu Pukki and Amido Balde both scored a goal each last night as the Hoops beat Trabzonspor 3-1 in the Antalya Cup in Turkey and that will have done both of Neil Lennon's Bhoys the world of good.
Would Celtic have done better in the Champions League if Hooper had still been at the club? I think there is a chance they might have.
In the opening group game against AC Milan in the San Siro they failed to score despite dominating for much of the match.
If they still had their English hitman then they could very possibly have netted an all-important goal.
I can appreciate why Neil is wanting to bring in one or two new players this month.
That would give them time to bed in and be ready for the Champions League qualifiers next season when they come round after the World Cup.
It would help him, too, if Celtic held on to important players like Virgil van Dijk.