Ireland star must come through "modified training" to prove his fitness for pivotal clash against Wales in Cardiff.
Ireland's lynchpin fly-half has "hit all his markers so far" in battling hamstring trouble, but is still fighting to recover to face Wales on Saturday, according to team manager Mick Kearney.
Number eight Jamie Heaslip was due to take contact training on Monday for the first time since fracturing three vertebrae in his back against France on February 14.
The Leinster stalwart is now expected to be fit to face Wales, where victory would leave Ireland almost nailed-on for the Grand Slam.
"Johnny Sexton continues to improve following a mild hamstring strain," said Kearney.
"As a precaution he will have modified training during the early part of the week with an expectation that he will train fully towards the end of the week.
"We are very happy where he is at, at the moment. He has hit all his markers so far.
"Johnny will have modified training in the early part of the week and if he comes through training through the latter part of the week, he will be available for selection.
"Jamie Heaslip is expected to train fully this week and barring any setback will be available for selection.
"Sean O'Brien and Jared Payne have both progressed well through the return to play protocol and will be re-introduced to full training during the week.
"Johnny will have modified training today. Jamie will train fully today."
Team manager Kearney admitted British and Irish Lions Sexton and Heaslip are both "extremely eager" to face Wales this weekend.
Ireland are gunning to retain the Six Nations title for the first time since 1949, and also claim a second Grand Slam in six years.
Sexton limped out of the latter stages of Ireland's 19-9 victory over England in Dublin on March 1 and has been battling to recover ever since.
The 29-year-old has a history of hamstring problems, but Kearney does not expect that to colour his recovery.
"From what I remember the last time he had a problem was between the Australia game and New Zealand game, in the November series just over 12 months ago," said Kearney.
"He actually recovered pretty quickly then.
"He came off at half-time against Australia and was fit to play and played the whole game, or most of the game against New Zealand without any issue.
"That certainly wouldn't be a worry at the moment.
"Jamie (Heaslip) hasn't taken any contact yet. He will train fully today and provided there is no reaction he should be available for selection.
"They are extremely eager, absolutely: very eager to play."
Fit-again Ulster lock Dan Tuohy was drafted back into Ireland's wider 36-man training squad for the final two weeks of Six Nations action.
Ireland must win in Wales and Scotland to secure the Grand Slam and retain their Six Nations title - but, barring injury, will do so without veteran centre Gordon D'Arcy.
The 35-year-old Leinster centre has been omitted from Schmidt's training squad, with Ulster's Darren Cave and Munster's Keith Earls providing midfield cover.
Team manager Kearney refused to accept D'Arcy's absence could spark the end of his glittering 81-cap Test career however.
His long-term centre sparring partner Brian O'Driscoll retired last summer, but D'Arcy himself has always been determined to push on to this year's World Cup.
"That is a selection issue, yes," said Kearney of D'Arcy's absence.
"Gordon has been an unbelievable servant for Irish rugby.
"He's back fit, back playing for Leinster, so certainly I wouldn't rule him out for future consideration."
Both teams must give consent for Saturday's Millennium Stadium match to be played under a closed roof.
Team manager Kearney said Ireland will delay their decision until later in the week, and base it around the chances of rain.
"We are waiting to see what the weather forecast turns out like," said Kearney.
"We don't have to make a decision on that until 7pm on Thursday.
"We will wait until then before we make a final decision.
"I think overall, our preference would be for a dry day and decent weather, rather than wet and miserable weather.
"In good weather our preference would be for it to be open."