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Gatland reacted angrily to questions on his "Warrenball" tactics, and said his side will play attacking rugby.
But O'Sullivan, part of the Lions coaching staff in New Zealand in 2005, says Gatland is "reacting to stuff he doesn't need to react to".
"He has certainly made a rod for his back now," O'Sullivan told BBC 5 live.
The former Ireland head coach added: "At the moment he is prickling at a lot of things, and it's very early days for that, it's a long tour.
"So I'm surprised a coach of his experience got sucked into the "Warrenball" debate. It has put pressure straight away on the team to put in a big performance against the Blues.
"It is just symptomatic I suppose of the pressure he is under."
Following the underwhelming 13-7 win over the Provincial Barbarians, Gatland cited jetlag and a lack of time in New Zealand as reasons behind the team's sluggish performance.
"The references to needing more preparation time is almost telling the squad that they are in trouble," O'Sullivan added.
"If he thinks they need more preparation time - which they can't change - it is sending the wrong message to the squad.
"It's something you might say at the end of the tour when you are reviewing it. It's very hard to put that out on the first week on the tour."
And with two crucial games coming up this week against the Blues on Wednesday and Crusaders on Saturday, O'Sullivan has warned things could only get worse for Gatland and the Lions.
"He probably needs to step back a bit and get on with it," O'Sullivan continued.
"If the New Zealanders feel they can go for him, they will go for him. This is off the back of being asked a fairly innocuous question at the first press conference, so I think he probably needs to just cool his jets a little bit."
The Lions' first Test against the All Blacks is on 24 June at Eden Park in Auckland, the venue for their next game against the Blues.
British and Irish Lions head coach Warren Gatland is sending out the wrong message to his players, says former Lions assistant Eddie O'Sullivan.