ONE thing every coach and player demands is certainty. Which is one reason why there is a lot of unhappiness around the Glasgow Warriors camp as they prepare to face Cardiff Blues at Scotstoun this evening.

Most of it has nothing to do with their Welsh opponents being one of the more dangerous teams floating around mid-table in the Guinness PRO12. Mainly it is about nobody having the faintest idea what to expect under the new tackle regulations that came into force earlier this week.

Most troublesome is the idea that a player can be punished for something that is clearly an accident. If there is contact with a opposition player's head then the hows and whys of the way it happened affect only the severity of the punishment.

Predictably Gregor Townsend, the Glasgow head coach, is worried about it. Not only does it introduce an element of lottery into a game his side need to win if they are to stay in touch with the PRO12 play-off battle, but it also makes things tough for the players who have had less than a week to change habits built up over many seasons.

"We are entering a real grey area," he said. "You just hope the game comes through it strong and with common sense. You know what it's like, you get one yellow card and it affects the nature of the game. When you get two or three or a red, the game is going to be totally different for the supporters and the players who have put that effort in. I believe they are coming from the right place so let's just wait and see."

The new regulations say any contact with the head has to be punished. If the referee decides it is accidental, it is still a penalty against the offending player; reckless gets a yellow card at best and possibly a straight red depending on how the referee sees it. Any element of deliberate contact is a straight red.

"The regulations that came out in November about players who are not part of the ruck and defenceless, targeting the head – it's brilliant that that is stamped out. It's dirty play and has to be taken out," Townsend added.

"The clips they showed then of deliberate targeting, they should be red cards. We're now entering an area where accidental tackles could end up as yellow or red cards. As coaches we will be learning and adapting. I really hope for the game that we find common ground that keeps the aggressive contact nature of our game and accept that the mistakes players sometimes make are sometimes not their fault.

"I'm really concerned about what the games could end up being over the next few weeks. There's going to be a huge level of interpretation on what constitutes an accidental tackle. The ball carrier could be doing things to initiate a high tackle by dipping in at the last minute. I just really hope common sense prevails. The best referees are the ones who have empathy with the game, what the player was trying to do and are able to sanction appropriately."

The game will be the first for a Scottish club under these regulations, though in theory they should be more of a help than a hindrance to Glasgow where defence coach Matt Taylor has made a policy of getting his players to tackle low anyway.

The problem is that there are some situations where it is impossible to be sure of avoiding an opponent's head and other positions where a player may milk a penalty by ducking into a legitimate tackle.

It all adds an extra air of uncertainty to what was an intriguing encounter anyway. Glasgow lost the reverse fixture earlier in the season – controlling large chunks of the game but struggling to break down the home defence.

"There are some big games coming up and we are just outside the top four [of the PRO12] now so we've got to do more than we have in the first half of the season to get there," Townsend accepted.

To achieve that he has again tinkered with the side that won last week, the most significant being the change at fly half where, after getting a run in a role a long way outside his comfort zone, Stuart Hogg is given a rest and Finn Russell is recalled. He partners Ali Price and outside him the Alex Dunbar/Mark Bennett centre partnership is also back in action.

Jonny Gray returns to captain the side and with Brian Alainu’uese banned, Rob Harley will be the second row cover on the bench.

"A few players didn't play last week. Coming back into the squad, they're really keen to play," Townsend explained. "We know Cardiff are a quality side. They have been saying this is a must-win game for them so it's going to be a real test."

Glasgow Warriors: P Murchie; T Seymour, M Bennett, A Dunbar, L Jones; F Russell, A Price; G Reid, P MacArthur, D Rae, T Swinson, J Gray (C), J Strauss, S Favaro, R Wilson. Replacements: J Malcolm, A Allan, Z Fagerson, R Harley, A Ashe, G Hart, R Clegg, S Lamont.

Cardiff Blues: M Morgan; A Cuthbert, R Lee-Lo, W Halaholo, B Scully; S Shingler, L Williams; R Gill, K Dacey, A Peikrishvili, M Cook, J Down, E Jenkins, S Warburton (C), J Navidi. Replacements: M Rees, B Thyer, S Andrews, J Hoeata, BJ Edwards, T Williams, N Robinson, R Williams.

Referee: A Brace (Ireland)