Šeško has not featured for United since the 3-2 win over Liverpool in early May, while De Ligt's spell on the sidelines stretches back even further, to the end of November last year.
Saturday's Premier League opener against Hull City looks likely to arrive a fixture or two too early for the Dutch centre-back, though there is more optimism over Šeško's involvement.
That is at least the impression Manchester United have given in Tuesday's update, with the Slovenian more likely to be in contention for the club's following fixture against Ipswich Town later this month than the trip to Hull.
United also confirmed that Mason Mount remains sidelined with the injury he picked up during the friendly against PSG on 8 August. Manager Michael Carrick had played down the severity of the issue in the aftermath of that match, but Mount has still not been passed fit to play as things stand.
The pair's return to training offers some relief for Carrick, who is preparing for his first season in permanent charge with a lengthy injury list still to manage. Šeško has not featured at all since limping off during the win over Liverpool, while De Ligt's absence stretches back to a knee problem that first surfaced in the closing weeks of last year and has kept him out of every fixture since.
Both players will now need to build up match fitness gradually before being considered for a first-team recall, with the club keen to avoid any setbacks so early in the new campaign.
United travel to newly promoted Hull on Saturday with kick-off at 1:30pm, looking to open the new Premier League season with a positive result away from home.