The Parkhead side rescued a point against Kilmarnock at Rugby Park on Saturday after falling three goals behind by the interval but found themselves still trailing leaders Rangers by 10 points and second-placed Motherwell by three points, albeit they have a game in hand.
Celtic flew in to Rennes at lunchtime today with four wins in the last 12 games and bottom of Group I on goal difference after losing 2-0 to Atletico Madrid in Spain and drawing 1-1 with Udinese at Parkhead.
However, Neil Lennon's assistant believes that if nothing else, the game against the French side on Thursday night can help the players get back on track for resuming their SPL title battle when they take on Aberdeen at Celtic Park on Sunday.
"It is a good time," said Mjallby.
"It is a good break for us mentally.
"We know that we have to win every game and you are expected to do so, especially domestically, and we haven't had the best start to the season.
"We are fully aware of that and we are looking for improvements in all areas.
"So this is a good break for us, to try to find some form and inspiration to bring out in the league as well.
"We want to play the best teams in Europe and obviously we are in a tough group.
"It is a challenge for the players to show that this is a level that we want to be involved in."
The former Celtic defender revealed the players who struggled so badly for the first 45 minutes against Killie have been warned against similar performances in the future.
"We had a good chat and obviously we showed them parts of the game and the first half was not good enough," said the Swede.
"If you want to play for Celtic it is not good enough.
"Defensively there were too many mistakes that cost us.
"But we can take heart from the last 10 minutes where we came back and showed character.
"I like the pressure so that is not a problem," he added.
"If you work for Celtic or play for Celtic you need to be able to handle pressure, that is the nature of the game."