Thompson could be recalled by Millwall '“ before returning to PortsmouthÂ
The midfielder has made a superb impact since arriving on a season-long loan from the Den in August.
Thompson has been one of the Blues' stand-out players during the League One title charge, featuring 25 times and scoring twice in all competitions.
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Hide AdThe Lions hold the option of recalling the Sidcup-born ace between January 5-20, however.
With Millwall currently in a Championship relegation dogfight, sections of the Fratton faithful are bracing themselves for Thompson to depart next month.
Harris admits he could cut Thompson's Fratton Park spell short.
But if that's the case, there's still every chance the 23-year-old could head back to Pompey before the transfer window closes.  Â
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Hide AdThe Lions boss told londonnewsonline.co.uk: '˜We need to react to what we can do in the transfer market as well. We certainly need to add to the group.
'˜We'll have to judge as we go.
'˜People think it is straightforward '“ they are our players so go and do what you want.
'˜But we have to work out what is best for us in the short term and long term for their development and learning.
'˜Even if we call back players next week it doesn't mean they have to be with us for the rest of the season.
'˜They can come back to us for a week or 10 days '“ train and play '“ then go back to their clubs.
'˜As long as they only play for those clubs (Millwall and the same loan club)Â then it's fine.'