The Hornets were leading Chelsea 1-0 on Saturday but conceded twice in the last 10 minutes to Michy Batshuayi and Diego Costa to come away empty-handed.
They will be back at Vicarage Road on Tuesday night to face Gillingham in the EFL Cup with Cathcart looking for a quick pick-me-up.
"Fitness might come into it a little bit, but it's still early in the season and we're all finding our feet," he told the Watford Observer
"The games come thick and fast so over the next few weeks I'm sure it won't be a factor at all.
"We want to mix it with the best teams and we did (against Chelsea)
It's hard to look at the positives because we feel so deflated losing those two late goals.
"But we'll look at the game in the next few days and look at the positives and pick ourselves up.
"I think we deserved at least a draw
I thought we were excellent in the first half, and restricted them to very little
"We got in behind a few times and in the second half we knew they would come at us and knew they had a lot of quality."
Boss Walter Mazzarri, still looking for his first win since taking over at the Hornets, could hand a debut to Younes Kaboul against the Gills following his switch from Sunderland.
But fellow new signing Roberto Pereyra is awaiting a work permit having completed his move from Serie A champions Juventus.
Summer arrival Isaac Success could be involved if he can shake off a hamstring problem but midfielder Ben Watson is still suspended following his red card against Southampton on the opening day of the season.
Source : PA
Source: PA