Celtic lost to Ibrox side and Old Firm rivals Rangers in the semi-finals of the Scottish Cup at Hampden Park in Glasgow on Sunday afternoon.
The game finished 2-2 after extra time, and Mark Warburton’s side clinched the contest 5-4 on penalties.
Stewart, who was at Rangers as a youth and could have joined the Gers as a professional on loan from United in 2004 (as reported by The Telegraph at the time), has explained the team selection that Deila got wrong against their Glasgow rivals.
“What Stefan Johansen was even doing on the pitch, ahead of a Kris Commons or Tom Rogic, I don’t know,” Stewart wrote in The Scottish Sun.
“Those were big calls and, I think, the wrong ones. Ultimately that’s what will prove costly for Deila but it is by no means all on him.”
Celtic were expected to win against Rangers, and their failure to do so means more pressure on Deila.
It is a bit harsh on the Celtic manager, as his side lost to the Gers only on penalties, but the problem is that the Bhoys have underperformed throughout the season and have not played particularly well either.