Chelsea are yet to taste defeat in eight Premier League matches under Maurizio Sarri this season, as the London club now sit level on points with joint-leaders Manchester City and Liverpool in the table.
Alonso was on the field for Spain in Monday night's loss to the Three Lions, in which Barkley was instrumental for the visitors third goal.
And Alonso believes Gareth Southgate could have done with Barkley in Russian Federation.
"If he can put it all together with his decision making going forward, he could be that creative player, that we said on the back of the World Cup, that England still lacked". He showed it not just today but against Croatia (last Friday) too: he showed that he can be a great player.
Barkley signed for Chelsea in January but struggled with his recovery from a serious hamstring injury and only managed four appearances across all competitions under Antonio Conte.
Barkley has played a hand in the Blues' early-season performances, with the former Everton man playing seven matches, scoring one goal and providing the same amount of assists.
The Spaniard added: "Last year he had a long time out injured, the first year with us at Chelsea and I have no doubt that with confidence and minutes he is a great player and I hope that he can help us a lot for the rest of the season".
With a successful worldwide break and his much-needed first Chelsea goal, Ross Barkley appears to be on his way to proving his move to Stamford Bridge from Goodison Park was an astute decision.