The reigning PRO12 champions fell at home to Newport Gwent Dragons for the first time since 2009 a fortnight ago, while last weekend's 29-8 victory over Zebre at the RDS was far from emphatic as they trailed 8-3 at half-time.
For McGrath, a professional response is required on Friday if they are to boost their semi-final hopes, as well as denting those of a side that is becoming increasingly familiar to the Irish province.
"We have targeted this four-game period and the last two weeks have not gone to plan really, but it's important this week that we start building," the 22-year-old told the club website.
"We need a big performance on Friday, it's very important for us young lads. We have to take each day as it comes, but we have to be switched on for training, knowing our detail.
"Every time we play Ospreys we know it's going to be so tough. We played them four times last year, so we know a lot about them.
"Every time we meet it seems to be very close, and no doubt it will be this week as well.
"Both teams are used to playing each other that we know every time we play them it's going to be tough, and similarly it's going to be tough for them as well."
Only two points separate the two sides ahead of the headline fixture, with the congested nature of this year's GUINNESS PRO12 table meaning both teams are within touching distance of top spot despite sitting fourth and fifth respectively.
And McGrath knows his side will need to be more clinical on the day if they are to keep pace with those at the summit of the PRO12 standings.
"We weren't accurate in the first half at all last week and in the Dragons game, but in the second half against Zebre we kicked on and scored four tries," he added.
"I think against a team like Ospreys you can't make the easy mistakes like we did in that first half.
"That is a big focus for us going into the match on Friday."
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Guinness PRO12
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