The Labour leader’s opponents said his actions had torpedoed talks on having some shadow front bench positions elected by MPs, rather than appointed.
Ms Thornberry said negotiations were continuing and Mr Corbyn should not be criticised for acting decisively.
Parliamentary Labour Party chairman John Cryer said he was not informed.
Mr Cryer said in a letter to MPs that the party leadership had not told him, or sacked chief whip Rosie Winterton, about the changes.
The PLP had held talks with party leaders over possible shadow cabinet elections.
A spokesman for Mr Corbyn said he was willing to continue the discussions on the idea of the party electing some of the shadow cabinet.
“On the one hand people criticise Jeremy for being weak and taking too long on his reshuffles and yet when he decides that he needs to do one in order to fill vacancies and reach out, people then criticise him for being too decisive and too strong. You can’t play it both ways,” she said.
“We stop fighting among ourselves”.
“We have a job to do. We were elected to be MPs, represent our constituents and stand up to the government. That’s what our priority ought to be and we need to get on with it.”
‘Not discussed or mentioned’
In this week’s reshuffle several MPs who quit the shadow cabinet in the summer in protest at Mr Corbyn’s leadership returned to the fold.
In other appointments, deputy leader Tom Watson was appointed shadow culture secretary and Jon Ashworth became shadow health secretary. John Healey returned to housing and Diane Abbott became shadow home secretary.
In his letter to Labour MPs, Mr Cryer said the PLP in early September voted “overwhelmingly” for the return of elections to the shadow cabinet.
“This led to negotiations involving myself and the then chief whip, Rosie Winterton, and people from the leadership team,” he wrote.
“As far as Rosie and I were concerned, the talks were held in good faith with the aim of striking an agreement which would allow some places to be filled through elections while the leader would retain the right to appoint others.”
Mr Cryer said it then became clear on Wednesday that a reshuffle was under way, which “had not been discussed or mentioned” during the talks.
“It now seems to me that the party’s leadership did not engage in the talks in any constructive way,” he added. “Obviously, I deeply regret this turn of events.”
Ms Winterton was sacked as part of the reshuffle.
Firm grip on power
But a spokesman for Mr Corbyn, who was re-elected as labour leader in September, said shadow cabinet elections were still to be considered by the party’s national executive committee.
Labour’s former home secretary, Alan Johnson – a former critic of Mr Corbyn – said he was still not up to the job of being leader of the opposition.
He told the Today programme “me and many of my colleagues” believed Mr Corbyn was not up to the job, adding: “Perhaps he’ll prove me wrong.”
BBC political correspondent Iain Watson said while some of Mr Corbyn’s critics had been welcomed back to the front bench, very few of his allies had departed.
The Labour leader’s hold on his party’s levers of power was firmer than ever, our correspondent added.