The Spanish champions returned to winning ways against Tottenham Hotspur in midweek, but go into this weekend's match having failed to win any of their last three La Liga outings.
It has been a tumultuous time for Valencia in recent seasons, with two successive 12th-placed finishes seeing them drop out of Europe before returning to the Champions League by finishing fourth last term.
Fans will be concerned that the early stages of this season have developed in a manner which suggest another of the disappointing campaigns, with Marcelino's side having won just one of their nine matches across all competitions in 2018-19.
Valencia are already seven points worse off this season than they were at the same stage of 2017-18, with most of those dropped points coming via draws. Los Che have shared the spoils in five of their seven La Liga matches this season.
On the bright side, they are unbeaten in their last five league games and last weekend they finally ended their wait for a first win of the campaign by beating Real Sociedad 1-0.
However, Marcelino will know that such regular draws are not a recipe for success, and the first port of call in order to turn those single points into three is scoring more goals.
Valencia have scored just twice in their last six outings across all competitions and drew another blank against Manchester United in midweek as they were held to a 0-0 stalemate at Old Trafford.
An away Champions League point is rarely a bad thing - particularly at a ground like Old Trafford - but while much was made of United's performance in that match, Valencia also offered precious little to suggest that they could grab hold of the game.
Los Che will certainly need more of a cutting edge if they are to pick up their first home win of the season at the expense of Barcelona, but they have only scored twice in their four home outings at the Mestalla so far.
Things are better at the other end of the field, though, with Valencia having kept three clean sheets and conceded just one goal in their last four outings.
Recent La Liga form: LDDDDW
Recent form (all competitions): DLDDWD
Barcelona can consider themselves very lucky to still sit atop the La Liga table going into gameweek eight following a poor run of form which has seen them fail to win any of their last three domestic outings.
Ernesto Valverde's side have dropped seven points in that time, but have been repeatedly let off by title rivals Real Madrid, who have also suffered their own slump in form.
Home draws with Girona and Athletic Bilbao were disappointing, particularly as Barca deserved little more in either case, but their 2-1 loss to Leganes will go down as the nadir of their recent blip - or at least they hope it will.
The performances have been poor as well as the results, but there were signs of a return to their best in midweek when they ran out 4-2 winners over Tottenham at Wembley.
Lionel Messi was in inspired form as Barca made it two wins from two in the Champions League - and eight goals in those two games as well - putting a Blaugrana back on the winning track after seeing their perfect start to the campaign ended a fortnight ago.
Sunday marks the beginning of a difficult run of fixtures for Barca, though, with Sevilla, Inter Milan and Real Madrid to come in their first three games back after the international break, so a failure to build on Wednesday's Wembley win could prove particularly costly.
Barcelona have lost two of their last four La Liga away games - having gone unbeaten in 21 such matches prior to that - and defeat at the Mestalla would condemn them to back-to-back away league losses for the first time since October 2015.
One area the Spanish champions do need to improve is defensively, having kept just one clean sheet in their last seven games across all competitions, conceding 10 goals in that time.
Sunday is sure to be a difficult test, with Valencia possessing the sort of pace which has caused Barca problems this season, but Valverde will know that their title rivals will not keep on letting them off the hook, so a swift return to form is imperative.
Recent La Liga form: WWWDLD
Recent form (all competitions): WWDLDW
Barcelona are sweating over the fitness of Luis Suarez, who is a doubt for this match due to a knee problem.
Should the Uruguayan not be passed fit then Munir El-Haddadi is the most likely replacement, although Valverde could also decide to switch to a 4-4-2 formation.
Ousmane Dembele missed the win over Spurs in midweek and is pushing for a recall to the side - which could see Philippe Coutinho drop deeper - but Arthur would then be expected to make way and the Brazilian put in a good performance at Wembley.
Valencia possible starting lineup:
Neto; Vezo, Garay, Gabriel, Gaya; Soler, Parejo, Kondogbia, Cheryshev; Rodrigo, Batshuayi
Barcelona possible starting lineup:
Ter Stegen; Semedo, Pique, Lenglet, Alba; Rakitic, Busquets, Coutinho; Messi, Suarez, Dembele
Head To Head
Barcelona have won all three meetings between these two sides so far in 2018 and are unbeaten in their last six across all competitions, winning five of those.
Valencia's past two wins over Barca have both come at the Camp Nou, and they have not won a home game against La Blaugrana in La Liga since February 2007.
Barcelona are unbeaten in 11 such matches since, but have only won five of those and drew on their last league visit in November.
We say: Valencia 1-2 Barcelona
This will be a stern test for a Barcelona side struggling for form in La Liga, but Valencia have also been patchy this season and Wednesday's Champions League win should restore some of the visitors' confidence.