Express Sport understands Southampton are preparing a £3million offer for the 25-year-old, while Everton and West Brom are also eyeing the playmaker.
Armstrong, 25, is in the last year of his contract. Celtic certainly want to get a new deal wrapped up as soon as possible, but the Scotsman may see this as his best opportunity to move to a bigger club and earn higher wages.
In 31 appearances last season, Armstrong registered 15 goals and 7 assists, playing largely in a central midfield role. The competition in Scotland is poor at best, however, there may be something to be said for the player’s versatility - Armstrong can deputize in attacking, central, or defensive midfield, and even spent some time out wide on the left.
For now, it would be difficult for Stuart Armstrong to break into the rotation at Everton. However, there does seem to be some talent here, and if James McCarthy and Ross Barkley exit, this is the type of player Ronald Koeman could be looking for as he seeks to fill out his squad this season.