Arsenal have had a £15m bid rejected by the Scottish champions but are expected to return with an improved offer.

Scotland cap Tierney, 22, is currently with Celtic in Austria for pre-season.

"I think it's probably now time for him to leave Celtic," former Liverpool player Adam told BBC Radio 5live.

"It's the next progression for his career. Seeing how well Andrew Robertson's progressed at Liverpool, as a player you've got to be thinking, 'I want some of that'.

"He's done amazing at Celtic, winning the treble treble. It's going to be difficult for him to leave because he is a big Celtic fan but if Arsenal do want him, they'll have to pay serious money for him."