Former First Minister Mark Drakeford says his Labour party colleagues made mistakes in how they handled negotiations over Plaid Cymru's summer mini-budget.
Plaid failed in the summer to get Labour or any other opposition party to support changes to its spending plans.
Drakeford said his party "did well" to secure more cash for children with additional learning needs, and suggested there was a case to be made to take the money.
Speaking at Labour's conference in Liverpool, Welsh Labour leader Ken Skates defended his handling of the supplementary budget, saying he was offered a "terrible deal".
The supplementary budget was worth £300m and included funding to reduce NHS waiting times and expand childcare.
Labour refused to support Plaid after demanding extra cash for children with additional learning needs.
Plaid said at the time it had offered £120m over three years, but Labour had wanted £100m to be spent in the current financial year.
Drakeford, who was first minister from 2019 to 2024, stood down from the Senedd last May.
In an interview with BBC Wales, he criticised the then-new Plaid Cymru Welsh government for "not being sufficiently aware of the fact... they're still quite a few votes short of being able to govern on their own, yet they set off to do that".
He said Labour did "very well to secure over £100m in commitments for additional learning needs".
Asked if Labour had made mistakes, Drakeford added: "Well, everybody's new, so I'm not going to say that we didn't make some mistakes."
Suggesting Labour could have agreed the budget, he said: "I think there was a case myself for taking that deal off the table while it was there.
"But I understand, there were pressures on that Labour group from our colleagues in local government, from our colleagues in the trade unions, not to agree it. I hope it will be agreed in the autumn."
Drakeford appeared at a Labour conference fringe event alongside former Counsel General Mick Antoniw who, in a recent article, warned that Welsh Labour in the Senedd was lacking "any clear ideological and strategic direction".
"This may come but it is not there yet. A strategy of oppositionism cannot in the long-term succeed in winning political support from the many former Labour supporters who abandoned the party in May," the former legal adviser wrote.
Drakeford said it would take the party a "little bit of time" to reorientate from government "and think what are we going to do the next four years when that's not our responsibility".
Welsh Labour was left with just nine seats after May's election, after dominating the Senedd since devolution began.
Asked at the conference if he should have taken the money for children with additional learning needs, Skates said: "Only if you want poor outcomes for young people."
He said: "We were holding out for more money because of schools, because councils, because unions, because parents, because young people, required it and called for it.
"We were absolutely right not to roll over to Plaid on that deal. As a result of that, they are now negotiating with the unions and they will continue negotiating with us.
"I think it would have been a terrible deal, an appalling deal for young people in Wales if we had accepted that."
Reacting to Antoniw's criticism, Skates said: "First of all, when it comes to ideology, we need to make sure that we put delivery first."
He said the article was "brilliant at contributing to debate" but "predates this conference when it outlined this huge, huge agenda for us to give people power back".