Pompey eye Miller
Pompey are believed to be monitoring the left-back, who has struggled for game time despite making his debut in December 2016.
Miller – who has one cap for Scotland Under-21s – spent the first half of last season on loan at Dundee before joining Ayr United in the Championship in January for the remainder of the campaign.
The 21-year-old is under contract at Celtic until next summer. Portsmouth have also taken former Millwall left-back James Meredith on trial as they seek reinforcements in defence.
Hayes set for exit?
Meanwhile, versatile winger Jonny Hayes, who can also operate at left-back, is another player who could leave.
The Irishman has played just 63 minutes across two games so far this season - the 3-1 win over Sarajevo in Bosnia, and the Betfred Cup tie with Dunfermline. He hasn't been involved in any league matches.
Zenit keen on Ajer
Zenit St Petersburg approached Celtic with a view to holding talks with defender Kristoffer Ajer, but were put off by the club's valuation of the 21-year-old, according to reports.
The Russian giants, who were linked with a £12 million move for James Forrest earlier this summer, were said to be keen on the Norwegian international but felt the Hoops' asking price was too high, according to Championat.