Nigerian striker Isaac Success and Argentine attacking midfielder Roberto Pereyra joined the Hornets from Granada and Juventus respectively and have put pressure on regular attacking duo Deeney and Ighalo, according to the Vicarage Road captain.
"Football moves pretty quickly," Deeney told Sky Sports.
"Obviously we have had two new lads in. It has given us a little kick up the backside to make sure we don't go too long without scoring.
"Last year I went ten games without scoring. Now I am two in five which is a lot better than last year.
"Me and Iggy [Ighalo], we are always going to be a partnership but we have got to make sure we keep scoring goals.
"If we don't there are two new lads that will come in and take our job."