The Gunners have slipped out of the continental elite for the first time under Arsene Wenger after stumbling their way to fifth in the Premier League last season.
A mixed opening to the current campaign has seen their credentials questioned once again, but they did get off to a winning start in Europe on Thursday.
Wenger made several changes for a 3-1 victory over Koln, but Hartson believes Arsenal need to take the competition seriously and follow the lead of Manchester United – who swept to glory last term to help counter a domestic standing in sixth.
“Chelsea, and Manchester United under Jose Mourinho of course, are a lot of people’s favourites for the title.
“But Arsenal can adopt Manchester United’s attitude to the Europa League last season.
“Mourinho said from the outset he thought it was the best way in for United – he was almost telepathic how he called that.
“Ajax hadn’t won an away game all the way through the competition, so it was almost as if United absolutely breezed the final it in a strange type of way.
“United had a couple of tricky away games, but they won it at a canter. I think Arsenal have to take it seriously.
“In the Europa League, they’re probably favourites to win it behind AC Milan.”
Hartson believes part of the problem for Arsenal is that they once again failed to bring in the necessary reinforcements during the summer transfer window.
With the squad still light in certain areas, the ex-Wales international has warned that an absence from the Champions League may become a more regular occurrence.
“They’ve lost two game in the Premier league already, they go to Chelsea on Sunday where they’ve not won since 2011, so that could be three loses in their first five games,” added Hartson.
“I know they’ve got this great record with qualifying for the Champions League – but last season was a wake-up call.
“They needed to do a bit more in the transfer window. They brought in Alexandre Lacazette but needed another defender.”
Arsenal head to Stamford Bridge this weekend sat 11th in the Premier League table, with six points taken from four fixtures to date.