Griffiths has started the last two games - his first starts in the Scottish Premiership since August - and rewarded Deila with a goal in Wednesday’s 4-0 victory over Motherwell, which sent the Hoops top of the league.
Guidetti has not got off the bench in either of those games as he continues to wait for his first goal since November, a barren run that contrasts sharply with his prolific start to his Parkhead career, which saw him net 11 times in 10 matches.
Celtic are believed to have agreed a fee with Manchester City to sign the striker on a permanent deal at the end of the season but the Swede is keeping his options open amid reported interest from Southampton.
Deila, who is “100 per cent sure” that Guidetti will complete his season-long loan, said: “We’ve been speaking with John for a long time but he’s decided now he wants to wait to see what his options are until March or April, maybe longer, so there won’t be any final decision this month. But we know we have him until the end of the season.”
The 22-year-old recently boasted he would hit a hat-trick in the QTS Scottish League Cup semi-final against Rangers on February 1 but he might not get in the team if Griffiths continues to impress Deila against Ross County in Dingwall on Saturday.
The former Hibernian player had been linked with a return to Easter Road but looks to have won over his sceptical manager.
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Deila said: “There is big competition up front and it’s also about the opponents we meet but Leigh has made improvement in a lot of things. He looks sharper now and needs even more confidence.
“I believe in him. I think he’s a very exciting player and his left foot is world-class, I’ve not seen any player have a better left foot in my life.
“He has skills but again you have to perform over time when you play at Celtic.
“I wouldn’t play him if I wanted him to leave.
“We made some demands of him and he has really, really worked hard to tune into that. He still needs to improve, everybody does, but he is getting to a new level.
“You saw yesterday how much he works for the team, how sharp he is, and he scored a very nice goal as well. So he is going in the right direction.
“He has played the last two games and we have won the games and he has played well, so he has in a better position than three or four weeks ago.”
With the likes of Charlie Mulgrew and James Forrest returning to fitness, Deila is pleased with his options, a situation highlighted by the fact that right-back Mikael Lustig struck two goals against Motherwell on his comeback from a hamstring injury after coming on for Adam Matthews, who netted against Hamilton in their previous game.
So he does not appear desperate to sign players in January.
“In January it’s hard but to keep the players is most important now that we have consistency through the season,” he said.
“We’re getting a better level in the team so it’s important to keep it. That’s more important than getting players in but if we can add some in as well then it will be perfect.”
One player Deila has missed out on is 16-year-old Martin Odegaard, who has moved from the Norwegian manager’s former club, Stromgodset, to Real Madrid.
Deila said: “I met him, but he had some options. It was tough choice for him, there were so many clubs.
“I gave him his debut at 15 years old. He is a special, special boy.”